Sounds like a codec problem. The
K-Litecode Codec Pack gets recommended a lot. I use
XP Codec Pack 1.3.0 for WMP and it works pretty well.
You could also d/l another media player.
VLC media player is a good one that has the codecs built in and will play most file types.
Go to this thread for more on info:
CODECs list for Movies /Alternate players / Video help forums