Go to Start > All Programs > Accessories and right-click
on the Command Prompt and choose "Run as administrator".
Type in: SFC /SCANNOW and press your Enter key.
If that does not work, try repairing your system. Boot from your Windows
Vista DVD, select the System Repair Option > on the setup screen, select
'Repair Computer > Select your installation of Windows > select Startup
Repair option and follow instructions.
My Vista Quickstart Guide: