I get exactly the same problem - on very rare occasions a video from Youtube or Vimeo will load enough to play but then it is very juddery, 99% of the time it just sits there with a black box and the 'timer' scrolling whilst it tries to do something with the video footage. The only difference is that I get it on a HP Pavillion desktop p6606uk fitted with 12GB RAM, I have a HP Pavilion dv7 laptop fitted with 8GB RAM and this is absolutely fine. Both computers run Windows 7 Home Pro.
On the Desktop I can replicate the problem using different browsers - sometimes using anything other than IE (9) (Chrome or Mozilla) it just refreshes the page and doesn't even try to load a video. Using the Laptop any browser works.