There are these things called codecs packs. Although there might be drawbacks to codec packs, the huge advantage is that you install just one piece of software that features more the codecs than you probably ever will need (excluding future codecs and versions of course). My personal favourite codec pack is the KLite Codec pack, which can be found here: http://home.hccnet.nl/h.edskes/mirror.htm
(while you're at that site, you may want to get Real & Quicktime alternative and VideoLAN as well). Upon installing this codec pack, (about?) all media players on your WIndows system should be able to play every format the codec pack features (and that's quite a lot). I like to use Winamp (http://www.winamp.com) for doing so, but other players like videoLAN, BSPlayer, Windows media player etc should be able to do the same.