I had this problem before, and I thought I fixed it. Before, I had a 5200 video card and I had messed up video, where the colors would become saturated and bright, and the picture garbled. I just took the hardware acceleration down a couple notches and it worked fine.
Now my real problem, I installed my replacement 6800 for the one I just RMA'd. It worked fine, I played a few games at high res and I played huge video files and DVDs all with full hardware acceleration on, everything was dandy. But today, I start up the comp and try to watch a video, and the picture was the same as when I had the 5200, all washed out, bright colors and barely watchable.
I assume this is a software problem, due to having this problem on two different cards.
Last time I checked, the Dxdiag told me that everything was fine, and I also have the latest 77.72 Nvidia drivers. What else could it be? Did I have to configure something when I installed the 6800? I just put it in, connected the molex connecters and I knew I had compatible drivers, what else is there.
EDIT: I have to set the hardware accelerations at "Disable all DirectDraw and Direct3D accelerations. ------------ Use this setting to correct severe prolblems with DirectX accelerated applications"