It sounds like a codec issue for you...

For XP this codec pac is very good
Download XP Codec Pack 2.0.8 - XP Codec Pack is one of the most complete codec pack

Or for Vista this pac
Vista Codec Package v4.4.7 Beta 4a (freeware)

You can also install the VLC media player, it has all the codecs pre-installed and can play just about all files...
VLC media player - Overview

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