I too have the same problem. All videos played fine until
(without installing or changing anything - at least as
far as I remember) the video window only shows a black
screen. Sound plays fine, thumbnails as well, if I drop
the file in VirtualDub it shows the video.
I reinstalled all available codecs, upgraded and updated
players, tried an restore point prior to the day the
problem started, updated video drivers and did anything I
could imagine except reinstalling windows.
The only video that plays correctly is coded with
quicktime from my digital camera.
I get this problem and I HAVE the codecs, explorer loads
the thumbs and sound works, but no video in any player.