DO NOT render directly to avi for Maya, the feature is there but avoid it at all costs. Its pretty cr*p. Use the link GSter posted as there is an entire post on how to render a movie.
The standard in the industry is to render frames and composite into a single file like mov, avi, cin etc... Think about it. All films are is a bunch of frames played at a set speed (24fps, 25fps, 30fps).
Render out the MR stuff to single frames at the desired res and away you go. Use VideoMach/AE/Premier/QT or similar to create the movie.
Chris (formerly R@nSiD)
When the power of love overcomes the love of power the world will truely know peace - Jimmy Hendrix
Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)