It does sound like the "Blaster worm" or one of it's variants. Although you have done a virus scan, the virus is most likely still there in your sys. restore files as these are locked off to changes for obvious reasons.
Right click on "My Computer" Icon, choose properties/system restore tab and turn off system restore. Reboot your PC and scan again for virus, if any found remove these then enable your system restore again.
Make sure you have the latest patches from Microsoft and your anti-virus is up to date.
I will look for the link from Spike on how to stop your PC from closing down while you do the up-dates unless someone else has it to hand and can post. j.