It sounds like a corrupt registry file.
Boot into Safe Mode (by pressing F8 during the startup process).
(Your screen will probably look rather odd, as the video driver won't have loaded. Don't let that bother you!)
Run System Restore (from Start > Programs > Accessories > System Tools) and select the option to restore the computer's registry to an earlier date. Choose the most recent date available which is prior to when the problem first occurred. (Don't panic when your computer shuts down and restarts; it's meant to do that!).
That will probably fix the problem.
Chris