I just helped my aunt with the same thing. Her harddrive had a bad spot, find out which brand harddrive it is ex. maxtor, western digital, seagate, etc, and go to their website and download their bootable utlities disk, run the full (the longer of the two) test, chances are that there is a bad sector on the disk.
That dosen't always mean that the disk is trash, you may be able to do a low level format, but you will loose EVERYTHING and you will have to re-install everything.
If it is a virius, I think you can boot from a norton AV disk, and that might fix it.
also try to hit down the F8 key right after the POST screen, when the choice comes up choose to boot into safe mode (If it gets there)
try that stuff
maybe your problem can be fixed before you just re-install. Sometimes that works, but if there is a disk, or virus problem, it will most likely pop up again, if you just re-install windows
If it does come down to re-installing windows, boot from the windows disk, and do a format, then reinstall a fresh copy, That will elimanite the virus issue