Anyone catch the Foo Fighters reference?
Commonly referred to as Desktop Aurora or Aurora Desktop, this long forgotten early Vista eye candy can be "hacked up" to work in the newer builds. It is not quite like the earlier builds but it sure it nice being able to run screen savers as your background. It was one of the things I always loved about xscreensaver under unix.
Read more about it
here and make sure to grab a copy of the AnimatedDesktop.zip program to automate the process for you.
You can also get it to work under XP although I haven't tried it personally. If you want to see some eye candy inside Vista without grabbing any files or programs, just open up a 'Run' prompt and type 'winsat aurora', hit [Enter] and enjoy.
The AnimatedDesktop.zip method is much more elegant though.