I had similar problems even after just recently upgrading my video card to an XFX Radeon 6770. I installed a few codec packs like ccc and ffdshow and the Matroska pack and also enabled gpu hardware acceleration and it seems to have fixed the problem. If after installing codecs again and checking your video player settings you don't fix the problem, I'd suggest reinstalling Windows after you backup your data. If there is something weird wrong in the registry or elsewhere in the OS it's better to spend a couple hours reinstalling than spend weeks troubleshooting in my opinion. Make sure you have the latest firmware and drivers for your PC as well including system BIOS, chipset drivers, and video card driver. I also recently replaced my system drive with a $100 90GB SSD drive and it's the best upgrade I've ever made. All scores over 7 now on the Windows Experience Index.