Quicklook

I recently bought a 24 inch imac from a friend of mine. I installed Snow Leopard on it. Though one thing is happening on it that didnt happen with my ibook (nor does it). my ibook will quicklook .avi files. where my new imac will not. a friend of mine has had the same problem on his mac book pro. any reason why this is happening or how to fix it? it seems like it has been only since i upgraded to 10.6. it shows the file info... but does not play the video