I'm sure someone more knowledgeable will weigh in and correct me if I'm wrong, but I am under the impression that AE CS4 does not play well (if at all) with the Athlon processor. In a couple of searches I found that an SSE2 enabled processor is required. I don't know whether that's part of your problem. I just found the word from the horse's mouth at:
looks like your right, my CPU does not support SSE2 and I dont think their is a patch so I can install CS4, but CS3 is still supported by my CPU. since adobe only has their latest trials available, does anyone know where I can get CS3 Trial? I dont wanna waste my money on something I dont want.