Sorry to put this here... There is a tool called GSpot. If you ever download a movie file that you cannot play because of codec issues, this tool will ID the codec needed so that you can download the correct codec. Almost every trailer video that I have downloaded needs a codec that I don't have, so I know this works great because I have never encountered a codec issue that this tool didn't solve.
Get GSpot here. DOOO IT DOO IT NOOOOW!