The exact same thing is happening to me. It started doing so lately. Never had that problem before.
I have the strangest thing happening to my system which started about 3 weeks ago:
When playing video (avi, wmv, etc.) the video will freeze but the audio will continue to play. However if I keep my mouse in constant motion the video plays smoothly, stop moving the mouse and the video stops. It's like my system/monitor/video card/whatever doesn't know to refresh the image on the screen unless the mouse is moving. I cam also press any key on the keyboard (constantly) and the video plays (refreshes) normally.
Here's some system info:
Nvidia 6800 GT
Obviously an older system but I have had no issues with it until this problem.