I almost tried this at one time, but then I hit a roadblock and didn't know what to do but..
I would love to see a program that can set an AVI or Mpg as either the desktop background..or XP logon screen.
Specifically I think that two things that would make good content come to mind. Some footage of the HL2 menu, or a looping video of a recording of rain hitting a window.
I suppose for the desktop background you could convert an Avi to Swf then create a webpage for it, but you'd have a tough time getting the resolution right, especially if you need to change it.
Any thoughts?