My son's computer got a similar virus one time and I found the main virus files at the root of my system drive. If you can boot into DOS mode, go to that drive and look for any files that have been added recently. Often they'll be put in the System32 directory as well. In DOS mode you'll be able to delete or simply rename them if you don't trust yourself.

It's generally a registry entry that calls one of these files and starts the virus. If the registry entry can't find the file you should be able to boot and get your a/v stuff working.

Your worst fear is it's some kind of rootkit entry. I think there's a guy in China though that's created a rootkit destroyer and I think it runs from a floppy or CD.

Good luck.

Art