Windows Essentials Codec Pack is a collection of Video and Audio Codecs and tools that are required to Encode / Decode (i.e., PLAY) Audio and Video files.
With Windows Essentials Codec Pack you should be able to play 99% of all the movies and music files.
It is free and user-friendly, plus it keeps your computer up to date with the latest codecs through W.E.C.Ps Audio / Video Codec Update Service.
Published under GNU GPL license, supports Windows 98 / NT / 2k / Me / XP / 2003 / Vista.