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Virtual machine software is incredibly powerful, but hard to conceptualize. Viewing a VM as a separate machine can make certain issues clearer.

I'm checking out the trial version of Parallels to run Windows XP on my Windows Vista system. So far, I like it a lot. Yet I'm concerned about viruses and surfing the net using my XP-"guest" virtual machine. I've searched online and can't seem to get an answer on if I need to install antivirus software on my guest machine as well? Also do I need to reinstall all applications on my guest machine? I see that I can share folders but can I also share apps?

Virtual machines are, to me at least, incredibly amazing pieces of technology. They're powerful in that they let you run multiple different operating systems on the same machine at the same time, as well as really being able to protect that machine from whatever happens within a virtual machine.

The problem, of course, is that it's very difficult to conceptualize what a virtual machine is.

The short version is consider your VM as if it were a completely separate physical machine, and make your decisions accordingly.

Let's see why that is.

A virtual machine or "VM" is a software simulation of an actual computer that you run on a real computer.

Seems simple in words, but harder to imagine.

So let's look at this picture of a part of my desktop:

A VM on my desktop

You'll notice that there's a Windows XP taskbar along the very bottom of that screen shot. That's the task bar of my Windows XP that runs on my desktop machine where I'm working.

In the background you'll notice a Firefox Window open on Google.

"... it's a separate machine that through the magic of virtualization just happens to share some hardware with the host machine."

In the foreground, however, you'll notice a Window that also has its own taskbar and start menu. That is a window to a virtual machine using Parallels Workstation that is also running Windows XP. And within that window you can see I'm running Internet Explorer, also open on Google.

The virtual machine acts almost exactly as if it were on its own physically separate computer. It has its own Windows, its own Start menu, its own software, registry, users and settings. It also has disk space allocated to it that it treats as its own separate hard disk. In fact, other than being hosted on my desktop, what happens inside this virtual machine is almost entirely unrelated to and separate from what happens on my desktop machine.

And that's by far the best way to think about it: it's a separate machine that through the magic of virtualization just happens to share some hardware with the host machine.

So, in light of viewing it that way, let's look at your questions again:

  • Anti-malware software: if this were a separate physical machine, would you install your anti-virus or anti-spyware software on it? If so, then you should treat the VM the same way. The VM does not get any protection from anti-malware software installed on the host machine.

  • Other software: if this were a separate physical machine, would you install your applications in order to be able to use them? If so, then you should install them in your VM the same way. The VM does not get any kind of special or shared access to the software installed on the host machine.

The point of confusion may be the fact that you can easily share files between your VM and your host machine. But think about it: you can share files between two separate computers as well, via networking. In fact, that's all that VM/host file sharing is - a form of network-like access between the VM and the host machine. The two remain as if they were two separate and distinct physical machines.

Virtual machines are incredibly powerful for certain applications if used properly. But by far the best rule of thumb to follow when understanding how they work is simply that they are separate machines.

Article C3725 - May 7, 2009 « »

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?

May 12, 2009 6:25 PM

I've been wondering, if I have files that I suspect have been infected by viruses, or that my virus shield (maybe falsely) reports as infected, would it be safe to run and use those files in a virtual machine environment, leaving the host computer unaffected? From this article it sounds like it.

Carefully, with the caveat that you would then have an infected machine on your local network. Make sure all your machines are behind a firewall with respect to this virtual machine. (Typically that would be a case for a software firewall on every machine.) And make sure that all other machines are up to date.

I would also take care with any software that attempts to share disks between the infected virtual machine and the host machine.

You might also consider disabling the virtual network adapter in the virtual machine.
- Leo

November 3, 2009 6:59 AM

Hope I'm not shunned for reviving an old thread here, but I read through Leo's rules and it doesn't seem to be frowned upon...

OK, I'm going to confess my IT n00b status in the area of VM's. While I'm an IT professional, I have yet to work with any VM software. I'd like some recommendations on what to use. Are there any open-source (free) solutions with a good history? I run Windows XP exclusively and prefer to avoid Linux (any flavor). Anyone have suggestions?

December 29, 2009 5:48 PM

Microsoft currently offers Virtual PC 2007 for free on their website:

VPC 2004 is available also.

March 21, 2010 9:39 AM

I use Virtualbox which can be found at It is simple, easy to use, and has low overhead on the host ;)

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