If you've got a problem playing movies full screen, but the same exact movie file plays OK at smaller windowed sizes, definitely suspect bad video card drivers or a slow video card.

Also, just because you get 5mbps downstream from your internet service provider, that doesn't mean the server that's feeding you the video stream is *sending* it at 5mbps. Perhaps that server is overloaded and/or is throttling its output?

Also, I've heard talk of some ISP's who deliberately throttle certain types of content. Although I've only heard about that in terms of peer-to-peer file sharing, it's conceivably possible that your ISP could be throttling multimedia streams as well.