Run the SFC /SCANNOW command. This will check and repair if necessary any critical Windows system files.
Click on Start/All programs/Accessories then
Right click on Command Prompt option and select 'Run as administrator'.
(or "Safe Mode" open a Command prompt)
At the command prompt enter: SFC /SCANNOW
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7 http://support.microsoft.com/kb/929833
How to analyze the log file entries that the Microsoft Windows Resource Checker (SFC.exe) program generates in Windows Vista: http://support.microsoft.com/kb/928228