Please try the following:
-For VISTA Users: click Start, in the start search field type 'cmd'. Right-click on the search result and select 'Run as administrator'.
For XP Users: Click Start and then Run, Type 'cmd' and hit enter.
-VISTA ONLY: If UAC is enabled, click 'Continue'
-Type: sfc /scannow
-Wait for the scan to complete (this can take several minutes)
-When the scan has completed, it should give you some feedback as to the result
Also, try running Windows Update to make sure you are fully up to date.