also if you suspect you are missing a codec needed to play a file then you should use a great little program called AVIcodec. it will tell you everyhting you need to know about a media file and more. plus its free.
this is a quote from there site:
"AVIcodec v1.1 - Free multimeda files analyser :
Gives detailled information, especially the codecs needed to play the file, and where to download those codecs if they are missing on your system."
you can download it here:
http://avicodec.duby.info/