I have the same problem. Just posted a thread about it (sorry)
I dont have a firewall or anti-virus. Vista's does the job
Its so annoying
As for the guy who said YouTube would stutter if his net went down, that is not true.
YouTube uses progressive downloading not streaming. It downloads little chunks of the video bit by bit and stores them on your computer for playback. If you want some real streaming try the BBC or SkyNews (not iPlayer)