AVI file format is a so called container format, the video itself in AVI can be coded in several ways. Missing video but audio OK normally tells that the needed codec to play the file is not installed on your system.
I use Shark's codec pack for Windows, it allows WMP to play almost any format video files: http://shark007.net/
Download and install the codecs, reboot and you should be able to play your AVI video.
Another solution is to install and use the free VLC Player. VLC has all necessary codecs built in, so it usually plays just about every possible video file.
VLC Player download: http://www.videolan.org/