John, 
Was the virus some sort of self installed program that you just can get rid of like a PC scan/protection thing that keeps popping up on the machine?
I have in the last 5 weeks had to repair 4 computers for friends with that problem. It seems that there is some type of malware out there that is a killer to a machine and once it's infected the only way to get the machine back is to do a complete reboot with the factory discs. 
There is a small partition on most PC's that is used for recovery but even those are  being wiped out so you are basically scr*wed trying to just reboot or reinstall just the windows again, it has to be done like a brand new machine again from scratch. 
Good news is the discs are usually about 23 bucks so it's not an expensive fix just a big pain in the @ss, still cheaper than a new hard drive too. 
Good luck with it and remember ... computers are you're friends. 

 So I am told. 
