This happened to me. Go to a youtube video on normal view. Now right click and select settings. Try ticking or un-ticking enable hardware acceleration. If not try using windows update and update all drivers. As this happened to me but after a while it dosent. So i think it was out-of-date drivers.
Yeah, I could get fullscreen with Silverlight on Netflix and with DVD players, but not with Flash (YouTube and Hulu), but disabling hardware acceleration worked with my ATI Radeon Mobility 7500. Thanks!