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

or for WMP
Get one of these pacs also...
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 v4.5.7 (freeware)

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