Ive had the same problem, windows media player is becomeing more obselete regardless of the version, even if it manages find the required codec it is rarley able to utilize it and play the file. I would suggest downloading DivX at DivX Video Player - DivX Video Codec - DivX Converter Its free and It will play many more files than WMP. The only problems ive had is the audio track will occasionally wander out of sync with the video.

Do you have the correct Windows player for your OS. I use WMP 11 with my Vista OS and have had no problems, but if say you are trying to use WMP 11 on ME or WMP 10 on SE they won't work. A friend of mine could get no sound on her WMP. Found out someone had put it on mute. Check to see that you have the correct version and everything is turned on properly. I have never had a problem playing any uncorrupt file with WMP on ME, XP or Vista.