When we expand the YouTube video it covers entire desktop area and all existing windows on the screen stay hidden as long as you are watching the video is this "maximized" state.
Here is a simple trick , just replace the word "watch" in the URL of any YouTube video with "watch_popup" and the video will expand to fit your browser window, not the desktop
For example original YouTube video is located athttp://www.youtube.com/watch?v=XkgMSCD-sDY
Change it to http://www.youtube.com/watch_popup?v=XkgMSCD-sDY and watch it
Thanks to Amit