Give us some more info on the computer itself. What operating system are you using? For instance, did the computer come with the current operating system installed or did you upgrade?
My computer did have the sniffles for a while, and I couldn't figure it out. It seems that whenever I get a virus it simply slips by all my defenses. Firewall, Virus program, etc.
When you say "sniffles" EXACTLY what do you mean? Are you absolutely certain the computer had a virus and if so, what virus? How did you remove it? What symptoms did the computer exhibit?
To be completely honest here, a virus doesn't generally "slip by" usually, the door is opened...either via opening an email from an unknown sender or opening an email attachment containing a nasty item, file sharing, using a computer without all the current security updates, using multiple anti-virus programs and/or multiple firewalls at the same time on the same computer at the same time, using rogue security programs which are actually created to ALLOW the nasties onto the computer and bring along it's buddies.
What anti-virus program, firewall, security programs are you using? Now I say generally because there are those times, occasionally, when the user IS doing everything right and a nasty will slip in. But frankly, I honestly believe this is really very rare.