(assuming you have Windows XP)
See if you can boot it up into Safemode. Then soon as you can log in, go to:
Start -> Run
In the Run window, type in msconfig
Hit the Okay button.
You have to do this before the bad program starts.
In the general tab of msconfig, you'll find the system restore button.
Use it to restore your system to a date prior to when you got the malware.
If you can't get that far, then let us know. They've gotten smarter these days.
Good luck!