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It's tempting to think more is better for anti-spyware and anti-virus software and firewalls. In reality too many can cause trouble.

With regards to firewalls, anti-virus programs and anti-spyware programs; can I have more than one of each of these programs installed in my computer? For example, I run ZoneAlarm; does that mean I should I turn off Windows firewall?

I've touched on this before but only addressed anti-spyware programs. The question is more common and more general than that, so in this article I'll tackle all of the big three: firewall, anti-spyware, and anti-virus.

Naturally, the answer on running more than one each of those three categories is different, and varies depending on exactly what you're doing.

As a general rule of thumb, you need only one of each. It's what I do. A single, good anti-spyware solution, an anti-virus solution, and a single firewall and, in my opinion, you're good to go.

However, as I've noted elsewhere, there's no single anti-virus or anti-spyware program that will catch all viruses or spyware. So there is a case to be made for having more than one. But if you do, you need to be careful because they can, and often do, interfere with each other.


Most anti-virus programs operate in two modes:

  • Scan: the utility examines memory and files on disk for traces of malware. This involves actually examining the contents each file for things that "look like" viruses.

  • Monitor: often referred to as "real time" monitor, the anti virus program is continually running and scans files as they are downloaded to your machine, notifying you nearly immediately if the file you just received contains something that looks like a virus.

There's nothing at all wrong periodically running an anti-virus scan with more than one anti-virus program. The key here is that it's just a scan - it starts, it scans, and then it's done. There's no opportunity to come in to conflict with another anti-virus program.

Real time monitoring, on the other hand, is another story. When you install most anti-virus programs they often automatically install and enable their real-time monitors. Running two or more real-time anti-virus monitors at the same time is very likely to cause a conflict. That conflict could result in error messages, crashes of the anti-virus programs, or other types of failure.

So it's certainly OK to have more than one anti-virus program installed, and it can make sense to run a scan using a different program from time to time, but you must make sure you only have one real-time monitor enabled at a time.

"Running two or more real-time anti-virus monitors at the same time is very likely to cause a conflict."

The simplest way to do so, as I mentioned earlier, is to rely on a single, good anti-virus program and make sure that its database of known viruses is continually being updated.


Anti-spyware tools operate much like anti-virus tools, and that typically means the same two modes:

  • Scan: the utility examines memory and the hard disk for traces of malware. While an anti-spyware program typically does not scan every executable file on the disk, it does involve checking certain registry entries, looking at the contents of certain files, and checking for the presence of others for things that "look like" spyware.

  • Monitor: like anti-virus programs, anti-spyware programs often have a "real time" component that monitors for certain spyware-like activities. A good example is that an attempt to change your default home page will be caught (or prevented) in real time by many anti-spyware programs.

And once again, the bottom line is the same: periodic scans by different programs are quite alright, while the real time monitors installed by these utilities can easily come into conflict. Make sure only one package has its real time monitoring facility enabled.

The most important thing is to start with a good anti-spyware program, and make sure that its database of known spyware is continually being updated.


Firewalls are a different beast from the tools we've talked about so far. They fall into roughly two categories: hardware and software.

A software firewall is just that - software that's installed on your machine that prevents certain types of intrusion into your system from the outside and, in some cases, monitors for suspicious attempts to connect to the outside from within your computer. In both cases the functions are performed in real time, as they happen. As you can guess from the previous discussion about anti-spyware and anti-virus software, two programs trying to perform the same action at the same time can lead to problems. I would most certainly not run two different software firewalls at the same time. That implies that if Windows Firewall is turned on, I would turn it off as part of installing another firewall such as Zone Alarm.

Hardware firewalls are, in most homes and small businesses, routers. Routers provide a level of protection that prevents your computer from being seen from the internet, unless you initiate the outbound connection. There's nothing to install on your PC; it's just another box that sits between you and your internet connection.

There's nothing wrong with having both a software and a hardware firewall. It's partially redundant, but it's harmless. In fact, if there are machines within your LAN that you don't actually trust, having both can actually be an appropriate choice.

Technically, there's also really nothing wrong with having multiple hardware firewalls. You can put a router behind a router if you like. However, a) it will slow down your connectivity somewhat, and b) there are certain types of communications protocols that may break as a result. Common protocols like web and email do not, so it's usually OK, but it's not really recommended.

Article C2833 - November 9, 2006 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

Vincent Mc Kenna
November 13, 2006 3:42 PM

This set me thinking ! I have been unable to install NAV for the last week, it keeps failing and asks me to start again, and keeps failing to install.
I notice now that Windows Firewall is " ON ", thats got me thinking -- thats good !! I will now have to learn how to uninstall Windows Firewall, etc. Thanks for the education.

December 6, 2007 12:59 PM

you have new messages

dana Jones
February 14, 2008 3:59 PM

I agree on the Firewall you can only run one firewall at at time. I use ZoneAlarm and it automatically turns off the XP firewall. As for AntiSpyware I have Windows Defender and SpywareDoctor running, and the PC does run a little slower, but the protection works. And for AntiVirus, on my new dual-core pc I have both AVG & Avast running with no problems. I chose AVG for the scheduler, and Avast for the boot-scan.

June 16, 2008 12:12 PM

Well, I use F-Secure Internet Security full time now (anti-virus, anti-spyware, and firewall) all in one package, but when I installed it, I forgot to uninstall Avast A/V and ran into all kinds of problems, (freezes, crashes), but that all stopped when I got rid of (uninstalled) the Avast. F-Secure disabled my Windows Defender & Windows Firewall by default upon installation and so far, so good.

bill g mosley
August 8, 2008 1:33 PM

i have both windows defender on, and symantic anti virus both running, does this conflict with each other

March 31, 2009 1:03 PM

I have an avast anti-virus (Avast) and windows defender/firewall in my laptop. I was planning to add either Symantec's Conficker (aka Downadup) tool, Trend Micro's Cleanup Engine, or Malwarebytes due to this conficker virus. should i go for it or would any of these conflict with my current protection or are my current system enough?

June 28, 2009 4:41 PM

It just means better protection

July 11, 2009 5:21 AM

Hi Leo,

When choosing a firewall what are main criteria it should match ??

How about NODE 32 SMART security, what's your opinion about it ??

Thank you.

John Doe
August 28, 2009 1:58 PM

If this author thinks that "as a general rule, you only need one of each", then he obviously doesn't have any experience with cleaning malware off of computers.

Armchair technicians for the lose.

Realtime protection is one thing -- obviously you only want one RT AV running at a time. But no single AV, AS, AA, or other AM program is 'all you need'. Any tech who has any amount of experience cleaning malware can tell you that no single program will get everything.

November 10, 2009 3:26 PM

Remember the key is you should not install nore than one real-time background scanning security program. Most people will do fine with just a simple hardware firewall (router), Windows Firewall, Windows Defender (Vista/7, Keeping Windows updated,having a real time basic virus scanner like Norton or McAfee and then using an on demand product like Malwarebytes for when you think there may be a problem.
Read about Windows 7 upgrades on my blog

Frank Williams
November 11, 2009 6:10 AM

I run 2 different anti-virus programs at the same time. I have run avg and Threatfire, and am now running Avira and threatfire. I left avg because it has become a resource hog. I have not had any problems doing this. Threatfire was designed to run with other anti-virus programs and works different than other programs and can catch viruses that have not been fixed yet. As I have only used it with these 2 programs I can not vouch for it running with any other virus programs. Check it out at you may change your mind about running 2 virus programs at the same time. I used this type of anti-virus program for many years as my only virus program. since the Dos days and have only had 1 virus in the whole time.

Glenn Ordell
November 11, 2009 4:31 PM

Mortal enemies??? Example: If you install Kaspersky and Spybot Search & Destroy, Kaspersky gets "mad" about Spybot. When you go to uninstall Spybot, their uninstall program asks you "why?" One of the listed options is: "Kaspersky."

Next, Spybot comes back with a paragraph explaining that they have thoroughly tested their product in conjunction with Kaspersky, and can NOT find any conflicts.

On the flip side of that coin, if you have Lavasoft's Adaware installed, and try to install Spybot, Spybot warns you that the two products are incompatible!!!

So who is speaking with a forked tongue?

Also, the rules of the game change on a daily basis. So two programs that were completely compatible yesterday may not be today because one of them was updated.

Hence, Leo's concept of not running more than one of each type of program is generally the safest bet. Exception: My computer was acted very strangely for quite a few days. Kaspersky didn't find anything wrong. So I "cheated" and installed Spybot, ran it, if found one problem, disposed of it, and now my computer's performance has greatly increased. Of course, I immediately uninstalled Spybot after running it to avoid any potential conflicts with Kaspersky.

Those are my thoughts...

Aloha from Hawaii,

Evan B Merz
December 15, 2009 7:45 PM

I have but one question: I have Vipre antivirus and it seems to be doing a very good job of scanning my system. Why is there never any mention of Vipre anywhere (at least where I have looked)?

February 28, 2010 10:45 AM

Hello, I found out that free antivirus, spyware firewall and so on don`t come complete, how may I assemble a package made up of free software: for example: avast free + zone-alarm firewall , and some other free peer to peer free protection and so on to be able to get it all as good as it gets without paying for commercial products (or downloading them illegally) ? Thank You! take care! Daniel

David Hutchins
March 16, 2010 8:29 AM

The new Panda Cloud Antivirus appears to run fine with VIPRE Antivirus/Antispyware/Firewall with no apparent conflicts or system slowdown. Also it is good to run an on demand manual scan for viruses once in a while with various products.

March 25, 2010 11:42 AM

can i run a virus removal tool with another antivirus??????????????

March 29, 2010 10:51 AM

As Leo reiterates- you can run two AV programs, but need to be careful because they can interfere with each other.

Immunet Protect ( is an AV that works with other AV programs; here's a list of compatible software:

June 20, 2010 4:49 PM

I am running norton 360 and Avast antivirus both at same time recently have been having a problem with down load speed suppose to have 8mps and only have 116kps could running them both cause this problem

Vic Pico
July 4, 2010 5:26 AM

using more than anti-virus program is possible.But your computer may run, slowly or stop, in some given time. is it my advise and opinion, to run on a reliable one anti virus.

May 16, 2011 7:31 PM

I am running AVG 2011 also have IObit 360 on here are they ok 2 run @ same time also microsoft security essentials is on here but its off as it wouldnt allow me 2 turn on defender i recently removed trojan and think it has done severe damage as my security centre wont always stay on i have 2 use Advanced system care 2 turn it bak on anny advice would be much apreciated as i am not only fixin my comp but i fix m8s comps a bit 2

October 14, 2011 8:25 PM

I'm using superantispyware and spybot. I'm thinking of adding in Windows defender and all together Avast would be there with the rest as my body guards.
Should I add in Windows defender? Is it worth it really?

January 2, 2012 3:51 PM

Microsoft Security Essentials installation instructions include the statement: " You can keep your antispyware programs running, as they coexist peacefully." Comment? True? False?

I would believe what the program is telling - why would it lie? It will also warn you to remove other anti-virus programs.
March 12, 2012 9:49 AM

For over 12 months, I've been running MSE along-side Avast, and using Search & Destroy for it's immunization and the odd scan.
When I had issues with Avast (v7 does not seem to run on my machine at all) and had to get rid of it, I soon managed to get a virus.
I've now managed to re-install Avast v6, I have had no problems sinse.
As a side-note, I have been thinking about adding Zone-Alarm's firewall to the mix, as I've used it in the past. Is it worth the extra effort to replace Window's firewall?

In my opinion, no.
December 26, 2012 11:42 AM

If I use the one Mfg's anti-virus software (tested as the best percentage in CATCHING viruses), can I use another Mfg's software (tested as the best percentage software in disabling/removing it)? And if so, must I somehow get the caught viruses out of the catching program's quarantine area or vault?

Amal Bhakta
April 14, 2013 5:18 PM

I am running Avast! (paid edition) and Microsoft protections (anti-virus, anti-spyware, firewall, ) simultaneously. The Microsoft protections are built into my IE 10 and Windows 7. Should I continue using Avast! or remove it from my system?

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