Sounds to me like a codec problem.
i would do this in order.
1. Remove all your installed codecs. via add remove programs
2. Update to the latest windows media player www.windowsupdate.com
3. go to www.free-codecs.com
download and install required codecs.
usually the ones if found i need are the Xvid and AC3 Codecs.
only install the ones you need. (just try those 2 for now)