Is this codec by any chance DivX? If it is then you'll need the software to convert it over into a file that can be played on WMP.

Go into this site http://www.divx.com/ and just download the free version. It comes with a movie player but you don't need to use it. When you download a video or movie with this type of codec this software will automatically convert it into an .avi, etc.

If it is a different codec then let me know.