Restoring the registry should fix it.
With your computer turned off put your bootdisk in the A drive, if you have one, power up and choose start with cd-rom support.
When you get to the 'A' prompt, type
scanreg /restore and press enter. (make sure there is a space between scanreg and /).
Now choose a date to restore back to before the problem occured.
Restart your computer.
If you dont have a bootdisk, you can download one
here.
Make sure you have a formatted floppy in the A drive and click on the file after its downloaded.