Media PLayer Always Black - no Video - HELP!

>
> When I play a video via Media Player, I just get a black
> media player screen. The sound plays, but No video.
>
> I just ran a test just to make sure I was clear on the
> facts:
>
> All formats show up black in Media player
> All .avi formats result in errors with Media PLayer,
> Quicktime and realplayer
>
> .mov and .qt play correctly in realplayer and Quicktime
>
> I just dont know where to start
>
> Is there a way to uninstall Media Player so that I can
> start with a fresh install??
>