Sorry, allow me to clarify a bit more...
Running two monitors on a 7800gtx with newest drivers. Video will black out when game starts up, but continue normally if I close it. I tried disabling overlay, but if I start a game thereafter the video freezes and the audio continues. Any ideas?
Firstly you might want to upgrade your nvidia video driver to the lastest.
And then you'll have to properly setup a multiple monitor with the driver's control panel, lets say you hook up a plasma HD TV to the other DVI port of your graphics card so you might want to change both monitors to independent resolution setup.
I've never tried doing video playback during 3D game play, it may or may not work properly, you might want to try out various video output modules, directx, direct3d, opengl, even wingdi...