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

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