Not sure what version of Windows you're running, but you can try typing sfc /scannow at the Start -> Run prompt in Safe Mode.
If that doesn't work, then you may need to reinstall Windows on top of itself to restore some system defaults, or repair corrupted system files. Ultimately, though, there might be some kind of infection requiring you to commit several hours of cleaning (using antivirus and anti-spyware apps). If that's the case, however, I would suggest just formatting and reinstalling Windows from scratch to save time.
~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
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