codecs are little bundles of software used to play certain file formats. For example, if you want to play DIVX files you need to download a codec for divx in order to play it. However if you want to play all media types, not just divx, try downloading ffdshow at:
sourceforge.net/projects/ffdshow
Also try Windows Media Classic, available to download at:
www.free-codecs.com/download/Media_Player_Classic.htm