That means that you need to download a codec pack.. it'll be a mix of most of the popular ones, divx, mpeg1, mpeg2, mp4, .mov (quicktime) etc etc. Anyway..
Get K-Lite or Nimo.. they're pretty good ones that should solve your problem.
You'll find them on this page (they're free, some aren't but that doesn't mean they're better).
http://www.free-codecs.com/Codec_Packs.htm