I'm working in 10.04 Lucid. All my video players (Movie Player and VLC) and the preview windows in editors (OpenShot and Pitivi) will only display a black screen when playing a video. The sound is fine and the videos work fine on other computers. 

I have tried multiple formats from varying sources .MOV taken from old ffmpeg projects, .AVI straight from a camera, .MP4 using h.264 from OpenShot on another system, .OGV from a gtk-recordmydesktop session on this very computer. I even get a pure black screen in the viewer when starting a remote session in VNC. 

This box has a GeForce 8400 GS using the proprietary Nvidia drivers in case it may be a card problem or setting.