problem watching YouTube videos

Hi I’m running Vista Home Premium with Service Pack 2. I have a 64bit system and lately I am unable to run any YouTube videos. I have no problem with DVD’s . I can go to YouTube, open the video, the commercial usually plays and then as the video starts it freezes almost immediately and disables my system so that I cannot access any internet sites and have to restart my computer.
Got any ideas what I might have done?

By the way, I have no problem watching the introduction video on this site! I'm confused!