First of all I would STRONGLY advise you to not do this. No matter how nice your computer is this is going to slow it down. You can set a animated .gif file as your background without and special software. If your dead set to have the video on there I would instead advise that you set it as your screensaver.
Heres a bunch of software that'll take care of what you want. Be sure and pay attention to the rating on the software, the number of downloads and the reviews. Good luck. Take your pick:
http://www.download.com/3120-20_4-0....round&tag=srch