Helping people with computers... one answer at a time.
Once infected knowing that you've removed a virus totally is theoretically impossible. In reality we most often assume that it can be done anyway.
How do i totally delete a virus on my computer? I have scanned and cleaned my computer when I got a trojan not long ago, but I still think it's just lurking in my computer.
It kind of depends on why you think it's still lurking. If subsequent scans still show that you're infected, that's pretty obvious, but if it's just that you're uncomfortable and don't know ... well, I'm not sure how to make you feel better.
Well, I shouldn't say that. There is, in fact, one way to make sure that you're no longer infected. In fact, to be completely honest, it's the only sure way.
But you're not going to like it.
The rule of absolute security is this: once your machine has been infected by anything, you can no longer trust it. At all. Ever.
The reason is that you have no idea what the infection did. What you do know is that the infection allowed someone with malicious intent access to your machine to do whatever they want to with it. The problem is that there is no way to be absolutely positively certain you know what they did, and thus no way to be absolutely positively certain that you've removed it. You have to assume that your machine is still "owned" by that malicious attacker.
That's both scary, and annoying.
So, the only way to know that you've totally deleted a virus is either:
Reformat and Reinstall everything from scratch. The operating system, the applications, all patches and updates, and your data. Everything.
Restore from a backup that was known to have been taken before the infection occurred.
The problem is that for most folks, either of those two approaches are impractical, or simply too much effort for the risk.
But if you're serious about security and need to be 100% certain, those are your options.
The more common approach is to scan with multiple up-to-date anti-virus (and perhaps anti-spyware) products until they all report things are clean. Yes, you do take on some risk that they missed something, but from a purely pragmatic perspective, nine times out of ten you're probably just fine doing this.
As you can see, this is why we focus so much on prevention of infection over recovery. Prevention, once in place, is significantly less costly than recovery from a problem.