Since everyone keeps suggesting it, I went ahead and tried to convert the MP4s. They failed to convert- the output file is the same. (However all players will try to play the video. Winamp wouldn't show a video screen, behaving like an mp3, while all the others (VLC, MPC) would play a blank black video screen. After conversion, winamp tries to play a blank black video screen.)
This suggests that something IS preventing the MP4s from getting through. However, I sent a couple of my files to a friend, and they play fine on their computer. So i'm stumped.
I've reinstalled the drivers twice, and also tried third party ATI drivers. Also tried older drivers. No difference (except the third party drivers made my games look wierd.) I've reverted back to the latest ATI drivers.
I'm going to remove my videocard and try the onboard video and see how that works. I'll keep you posted on the results.