Do System Restore in Safe Mode, if unable to do it in Normal Mode.
Try tapping F8 at startup, and from the list of startup selections, select Safe Mode by using UP Arrow Key to go there > then hit Enter.
Try a System Restore once there, to pick a Restore Point before your problem..
Click Start > Programs > Accessories > System Tools > System Restore > pick a different time > Next > etc
Cheers.
Mick Murphy - Microsoft Partner