Just in case any windows 7 users haven't clicked on the themes link I figured I would post this:
http://windows.microsoft.com/en-US/windows...=desktopgadgets
The bing theme is pretty cool, it has like 20 different images that they use for the search engine. I set my theme to that and put it on shuffle and have a different desktop every time I start up my computer.
Oh and their are more "gadgets" on there too.