surfasb essentially has the right idea. Once you have one virus, it's already downloaded all its virus friends and they download updates to avoid virus scanners.
You can try to remove the viruses but you'll never know if you really got rid of all of them or if there are still some malicious settings left over.
The best way to know there are no more viruses is to reinstall.
If you still want to try to remove the viruses, you can try this:
There is some pretty good information here that you can try.
Leave the computer disconnected from the internet for a week to keep the viruses from updating. Take the infected hard drive out of the computer and hooked it up to a known good computer as a secondary drive. Update virus definitions on your clean system and scan the infected hard drive. Most antivirus products have an option to do a custom scan which would let you just scan the attached (infected) drive.
I've had good luck with SuperAntiSpyware. There is a free version. Worth a shot. The more antivirus programs you scan with, the better.
Stick the previously infected drive back in the computer, re-install antivirus (it might have been crippled). Make sure you have all the latest windows updates and service packs.