It may be that some corruption has occurred and relevant files are damaged.
There are two things that come to mind that you could try-
1- See if you can do a restore, maybe the problem occurred after an update?
2- Try running a scan/repair using the SFC /SCANNOW command. This will check and repair if necessary any critical Windows system files to do this click on Start/All programs/Accessories then right-click on Command Prompt option and select 'Run as administrator'. At the command prompt enter: SFC /SCANNOW
You might also have a look at- click here shows how to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows 7.