you don't set the image format to .avi. You set it to .tga or .tiff or something. That will give result in a sequance of images. If you have 90 frames, you'll have 90 images. You'll then take those into one of the programs that we mentioned earlier or another one and create an avi with it.

As for it only rendering the one frame, try Optimizing your Scene size (File > Optimize Scene Size). If that still doesn't work, I've seen that selecting all your objects and exporting them to a new file work in such an occurance. If neither of them work, I'm not sure.

Remember, though, that selecting an .avi format will only render one file.... an avi file. Maybe that was what it was doing.

Good luck!