So from what I can see, it seems to be a 64bit issue. I just tried the Ubuntu 32 live cd and everything, and I mean everything, worked well. Streaming DivX and Flash fullscreen (same vids as tested on other OSes) Even my avi from my external hardrive was streaming fine in fullscreen mode.

So, what does this mean? Do I have to remove 4gb of RAM or can I make this work?

Thanks for all your help so far!