shut down the pc and re-start it , tap the F8 key untill you see the OS start menu,
select boot to safe mode
in safe mode open command prompt:
sfc /scannow
the file system check should repair corrupt system files.
Note: if that even fails then boot to a windows 7 OS disk and under repair my computer, it also gives you access to the command counsel and you can all so run sfc from there to.