It's all formats Divx, MKV etc. Put FFDshow x64 on and AC3 filter when I built the machine about 6 months ago, Had to to get H.264 playing through WMC. It's only recently it's started to sync out, Looked on the net and it does look like a common problem. So much so, people are going back to Vista for WMC, VLC player is far superior in audio and quality. It's a shame VLC don't do media centre's I wouldn't touch WMC. Un-installed FFD to see if that was the problem, but No!
Steve