Could you hold down the WindowsKey and press R (then let go)
and type:
sfc /scannow
(make sure you have your Vista disc ready if needed)
Are you using a piece of software called"Get Right"? or software by the company "Headlight Software"?
Have you installed any Codecs software? If so, please uninstall it.
Do you know when the problem started?
If so, you can press the WindowsKey on your keyboard and start typing:
system restore
Click System Restore, and run through the wizard, restoring to be point before when this happened.