I've thought about this before. I found a way that works, except that you must use a spot light and that produces a cone instead of a straight beam. I tried to work around by pulling the light way back and narrowing the beam, but that didn't help much. It would be nice if Carrara had a straight square beam like Bryce (which I haven't used in years. Also the light has to be perpendicular to the window, altough I think I may be able to get around that. This is a straight Carrara render, no post. It's late and I'll describe how I did it tomorrow.