Now, this might seem like a question almost not worth asking, but have you completely restarted since this issue?
(Vista users are being suggested, with such great power saving technology, to leave their computers in Standby. Thus, why I asked)
Vista will usually correct this issue on reboot.
(Broni: Would sfc /scannow work for such a situation of a file being corrupt?)
( The Buggles - Video Killed The Radio Star ... what a great music video)