You might try booting into "safe mode" by hitting F8 while your PC is starting Windows, but the bad guys probably have their hooks in too deep for you to kill their program even in safe mode. You have to keep the virus from starting in order to be able to do anything about it.
Get a friend to download something like the Trinity Rescue Kit and burn a clean CD. It's a self-contained operating system that lets you edit the Windows registry and run four different kinds of antivirus programs while you're booted into a clean non-Windows operating system.
http://trinityhome.org/Home/