Is it possible to have an animated wallpaper for a Windows 7 64 bit computer?
Windows 7 supports video backgrounds and even offers some samples of their own.
UPDATE new (old) info: Dreamscene requires the Ultimate Edition of Windows Vista. I don't run it myself but had when I used Vista and had conflated memories in my mind. Apologies.
However, you can still install Dreamscene in Windows 7, though it does require registry edits and such: http://windows7center.com/tutorials/how-to-enable-dreamscene-in-windows-7/
This solution will allow you to right-click on .mpg and .wmv files (any video file that will play in Windows Media Player) and select "Set as desktop background" and the video will play on endless loop.
In my experience with Vista, this did not slow the computer down so long as the computer was capable of handling Aero effects easily.