I've never heard of using an avi as a background, however, you can use avi's as materials.

So, load a 1 sided square as a BG, and set it's material to your avi. Make sute to click the little "key" in the advanced materials on the texture to enable "Animated".

If there's a "key", it's animatable (and keyframable).

Here's an example I used with an avi transparency.

                                                                                                                    

Poser 10

Octane Render

Wings 3D