Start > Type CMD > Press Enter
Does CMD (command prompt) window load?
Type Sfc /scannow > Press Enter
This is to see if it is working properly or if you receive an error message.
The scan will take a few minutes
Use the System File Checker tool to repair missing or corrupted system files
http://support.microsoft.com/kb/929833/en-us
Have you recently scheduled a New Task on the computer?
If the system was fine two days ago or so, perform a system restore to earlier date/time.
http://support.microsoft.com/kb/306084/en-us
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