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

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 v4.4.7 Beta 4a (freeware)

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You die for nothing...