Vista and MCE, as well as the WMP11, have an issue with codecs. Primarily, few are installed. And good luck if you want to play DivX or Xvid.
There is a fix thought, get FFDShow Codec pack from SourceForge.
don't bother with KLite, or AllIN1, because they will cause you more problems. Most of thier codecs are out dated, and half of them, in my experience, ar corrupt.
FFD merges with WMP11 and MCE, and will play most movie types.
If you ahve the wrong codec pack you will have a couple issues, Run32 DLL will halt so will Com Surrogate when you open your media file. It will cause MCE to lock and shut fdown as well.

Hope this helps