Most likely a codec problem. The audio is probably encoded in a standard format but the video is encoded with something wacky.
The thing about .AVIs is that it is just a shell; inside an AVI file you can have any number of codecs. Your video can even change codecs a few times per second to best adapt to whatever happens to be showing on the screen. This makes AVIs the most versatile and the best, technically speaking, video codec -- however, it is the hardest to use as you can't just download an "AVI Codec" since that is essentially "every codec on the planet."
So. You will have to find a codec analyzing tool to find out what codec your video is in, then download that codec.
If it was a video download from the web, it's likely DivX. Go to DivX.com and download their crap.
If it's from a camcorder you might have to pop in the camcorder CD and use the proprietary codec from there.