Lady Gaga? That's your reason
On a serious note; your system should be able to play 1080p. Try another video on youtube in 1080p (e.g. new divide by linkin park). If it still doesn't work, try playing a 1080p movie file on your PC (e.g. H.264), there's plenty of 1080p test files out there (just search google).
EDIT: re-read your post and your CPU is....well, ancient. I wouldn't be surprised if that's the reason. Upgrade your CPU.