It sounds like you have the blaster worm.
You might be able to keep things up long enough to get it out of there if you reboot in VGA mode. For some reason that worked on my Dads PC. If not it gets harder.
If you google "blaster worm" you should find lots of places to help.
Microsoft Corporation has information on it.
If you can post the exact message you're getting then we might be able to confirm the blaster