It's your graphics card. Make sure you don't have AA turned to the highest setting, and make sure you don't have an extreme resolution set. I have seen others SLi these card, and still have to being on medium/low settings.
I would suggest downloading VLC Player, or Windows Media Player Classic : Home Theatre.
I was experiencing similar issues with h.264 playback as well, and I used these video players instead, and have had crystal clear screens no problem.
First time post, and I have to say your forums are absolutely amazing. You guys really know your sh!t. So hopefully you can lend me some advise or suggestions.
I recently updated to Windows Se7en Build 7068 x64 and everything has gone swimmingly well. Only speed bump I've ran in to...