I am running Firefox 3.6.6 on Windows XP SP2 on a Dual Core. Why does my CPU usage increase up to 100% for one core when I watch movies on youtube.com?
I have noticed that some versions of flash seen to do this. You might try upgrading to the latest and greatest version to see if it helps. Another thing that might affect this is the quality of the video you are watching. Many older systems just don't have the power to render decent video.