I downloaded a movie whith avi format. But when i go to play the movie all i get is the sound and no video? By the way wat is avi format? Thanx

Audio Video Interleave is a basic Microsoft format for bundling audio, video and
information tracks together into one time-synchronized file.

Get this thing and use it to analyze your AVI, and it'll tell you the name of the
video decoder you need: [GSpot Codec Information Appliance]

Another option would be the VLC player at http://www.videolan.org/ which has codecs' already built-in and should be able to play almost every movie.
However, Aarons link and suggestion is definitely a "must" for people downloading movies. I use it by myself and it has helped me a lot

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