AVI files won't play in Media Player

I have an avi file that won’t play in Media Player. All other files I have been able to trim and play in WMP, but this one, for some reason won’t. The error message reads: “Windows Media Player cannot play the file. The Player might not support the file type or might not support the codec that was used to compress the file." Any idea how I can fix this? It is an episode of Friends, titled “5_11_Friends.avi”

There’s many possibilities:
1. The file was encoded using a codec not installed on your computer and you’re missing that codec to decompress the movie. To find out which codec you’re missing, try a diagnostic utility such as gspot (http://www.moviecodec.com/downloads/42d.html) or avicodec (http://www.moviecodec.com/downloads/43d.html)
2. You have the codec, but the avi is truncated (meaning it’s too short - this can happen when a download is incomplete). Gspot will also alert you of that (it will say corrupted avi header). In that case use an alternate video player such as media player classic (http://www.moviecodec.com/downloads/307d.html) or avipreview (http://www.avipreview.com).
3. The file is not really an avi, but another container (wmv, mpeg, mov, whatever...) renamed. One of the diagnostic utilities should be able to tell.
4. The file is a fake (aka not an avi). Gspot should be able to tell (it will say non-avi or unknown format).