You need some codecs. For XP try: Download XP Codec Pack 2.0.7.1 - XP Codec Pack is one of the most complete codec pack

For Vista try: Vista Codec Package v4.4.5 (freeware)

Also try VLC media player, it has the codecs built in and is very reliable. I keep it on hand just for files that won't play right in WMP. It's a good back up for other players.

More info: CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers

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