My macbook won't play video for avi files, just sound.

I got a new camera that works great with my friends aluminum macbook but my black macbook will not play the .avi video, only sound, but pictures work fine.
Also in Iphoto it says "Quicktime and a decompressor are needed to see this picture"
I have downloaded perian, div x and xvid and no change?
This is so frustrating, any help would be greatly appreciated!!!