If you haven't already resolved the problem, did you consider the possibility of a codec problem? Have you tried using VLC player? you can get it at
Free Software Downloads and Software Reviews - Download.com
If it's a codec problem, you can go search for codecs like Cole2K or an updated Windows Essentials (something something) for WMP...
Here's a site with loads of video codec for download:
ETPlanet.com - Download Section - Files
Hope this helps...Sorry if this came too late or wasn't helpful...
Good luck!
BTW, Cole2K Media Codec and that WMP Essentials thing allows you to play MKV, in case I forgot to mention
