DEEJEMON  It's probably not Xvid then. Get Info on the movie and check the signature for the video. There's usually a 4 character code that should give you an idea of what video codec is actually used.

.AVI does not imply any particular codec for either the video or the audio. It's a container, like QuickTime. And into that container, you can put video encoded into any format, audio encoded into any format, and other things as well. .AVI doesn't tell you anything about what is actually needed to play it (even if Windows people think it does).

To get your file playing, you should also try VLC Media Player. It handles a lot of file types and a lot of codecs. It is also less picky about looking through the container and finding video/audio it can play. If QuickTime doesn't find video/audio where it expects it inside the file, it tends to just give up; VLC is much more likely to keep looking and find something.  
(Version 0.5.1)