We've done this to one of the executives at work before.....
1) Take screenshot of desktop (print screen button + paint) and save it
2) Move all (movable) icons on desktop to a temp folder
3) Tweak windows so that it doesn't display any special icons (ie. Internu Exploder, My Docs, etc.)
4) Set the background as the screeshot you took.
5) Laugh as the (l)user tries to click on his/her "icons".