Odd Virus Situation

We decided to give our old 600mhz E- Machine compute to Connie's sister. I never use it. It just sits there except for every few months when I decide to update Windows XP and the Zonealarms firewall.

So, I'm uninstalling a bunch of programs I know she doesn't want on the computer and the anti- virus program starts doing a scan. When I notice it's scanning I go to turn it off as I don't see any reason to scan as I never do anything with it. How could it possibly have a virus?

Lo and behold, when the window pops up showing scan results it shows it found this virus. I end up having to delete it as it won't let me repair it.

How strange. All I do with that computer is download updates. I would think the virus would have had to either been hidden in an update, or, someone snuck it through my firewall.

I guess I'll check my laptop now. Maybe the same thing happened to it?

Update: The laptop is clear of any viruses. So, I go back to the E- Machine and redo the virus scan since I stopped it right after it started the first time. The scan showed an additional 25(!) of the same virus in various other files. Youch! They've been taken care of now, but how did they get on there?