Helping people with computers... one answer at a time.
Sometimes it seems obvious, sometimes not, but ultimately there's no way to prove that a computer is not infected. Best we can do is increase the odds.
How can I tell if my computer is infected? I picked up a bunch of malware from face book. I have run several programs and erased about 15 trojans. AVG says I'm protected. Is there a program I can run to make sure I'm clean? Computer appears to be running fine.
No one's going to like this answer. Not at all.
I'll start by putting it a slightly different way: while there are many ways that you'll notice some ... many ... perhaps even most infections, there is no way to prove that your machine does not have malware.
You cannot prove that your machine is clean.
Sounds scary, and I guess it is. So I'll also discuss why I'm still using my computers every day while still sleeping soundly at night.
You cannot prove that a computer is not infected.
Or as the semanticists or philosophers or perhaps plain old pedants would say: you can't prove a negative.
But I'm no philosopher, so I'll put it in more concrete terms.
There is no tool, no scanner or collection of scanners that will prove you do not have an infection. No tool catches everything. Even if you run them all, they're all as up to date as possible, and they all come up clean ... there might still be something.
There's simply no way to know.
Now, before we all throw all our computers out the window in a fit of collective paranoia, we also need take a more practical, pragmatic perspective on the situation.
After running all those scans and having them come up clean, it's pretty darned likely that your machine is in fact clean.
We just can't prove it.
So, how do you tell that your computer is infected? You might be infected if ...
if your good, up-to-date anti-malware software tells you you are, you might be infected.
if your computer's performance is suddenly affected, you might be infected.
if your internet speed is suddenly affected, you might be infected.
if you're suddenly getting popups telling you that you need to download and run some scanner you've never heard of, you might be infected.
if your machine suddenly won't boot, or keeps rebooting before you can log in, you might be infected.
You get the idea.
Note the annoyingly repeated use of the word "might" in all those symptoms. That's because if any of those symptoms appear it doesn't mean that you are infected, it just means that you might be infected. Malware detection and removal should be part of your diagnostic efforts.
Here's the kicker, though:
if your machine is running cleanly, quickly and without any apparent issues ... you might be infected.
You're probably OK, but ... you might be infected.
So what do you do? If even an apparently clean computer might still be harboring malware, what do you do?
You stack the deck in your favor. You increase the odds that it's not infected. You'll never be able to prove it's not infected, but by following some basic, common, and often repeated steps, you can dramatically increase the likelihood that it's actually a clean machine:
keep your software up to date to make sure that any discovered vulnerabilities are corrected on your machine
install and run up-to-date anti-virus and anti-malware software, and make sure that they are updating their databases of information daily
get thee behind a firewall - software or hardware using a router
be careful who you share with and connect to - particularly in your home and on your home network - one compromised computer or uneducated user on your local network can wreak havoc on all your machines
don't be stupid.
I don't think I've ever put that last one so bluntly before, and I don't mean it to offend, but I want to get your attention.
Recent reports of the number of people who regularly click on spam, download unsolicited attachments and just generally disregard even basic security has me asking what the heck are they thinking?
If you don't know what I mean by "being stupid", that's ok! Take it as an opportunity to educate yourself on the basics of safe behaviour on the internet.
The people that really concern me are those that do know, and yet act stupid anyway.
All the tools and safety measures in the world can't protect you from yourself.
That we can prove.