What says it is broken?
There is a program called "avifix" that can be used to repair faulty avi header information.
The program "mediainfo" will display detailed information on video files.
You could use ffmpeg or mencoder programs to transcode the file. If the problem has to do with the container format, but the video & audio are OK, you can use ffmpeg to copy the streams into a new avi file.
ffmpeg -i infile.avi -vcodec copy -acodec copy -y outfile.avi
if you don't have audio:
ffmpeg -i infile.avi -vcodec copy -an -y outfile.avi
The extension (.avi) only indicates the type of container. The video & audio inside it may use different codecs. You may need to use ffmpeg or another encoder to re-encode the video stream using a codec that works with the program you are using, or that your friends can view. Xvid is a very common one. Even my DVD player will play xvid encoded avi files of a CD.
Look in your package manager, as there likely several ffmpeg or mencoder GUI programs available. You may find that easier than encoding from the terminal.