There is a small utility GSpot, (
http://www.headbands.com/gspot/) by running, the program tells you which codec the file uses. Then according to the information you get, you could download the codec. The program runs on MS OS's. I don't know such an application for other OS's.
One good thing to have the VLC installed (
http://www.videolan.org/vlc/) installed on your system. If this fine utility can't play your video file, then there is a possibility, that your file itself has problems, not a codec issue.
Both of the programs are free, and VLC has many versions for many different OS's.
I would start with VLC, to make sure that the file is OK, and then move on to GSpot.
Hope it helps