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).