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 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 people, 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 would always focus on prevention as its cheaper than a recovery from a problem.