well, I have had this probelm on both 32 and 64 bit machines. It is definately a combination of video hardware/driver
issues. My previous media centre machine worked fine until intel release a driver update that prevented the display on SOME video files encoded with divx but not all.
I checked different versions of divx encoding but never found a full solution. unfortunately the crap minis dont have a good/updated video. I never had this problem when running media centre via my nvidia based machine to the plasma. (on my old 7800 and now on an 8800).
I have since tried some reg hacks from xp/vista but they dont seem to make any difference to the acceleration settings in Windows 7. It is probably something to do with the driver now. Fat chance getting Intel to update whatever they need to update to fix it.
I am currently using vlc to play media from my NAS device but I really like the media centre interface. There isn't anything that can beat it at the moment. I have been using it since XP Media centre edition was released a long time ago.