We have pulled that one at work several times!
Even in our IT department it fools guys!
The best one we ever did was turn our boss's PC into a Cher fest. he admitted to us that he was taking his fiancee to a Cher concert on her farewell tour. He was not happy about it
So..... We got some Cher wallpaper and even loaded mp3's on to his PC to start playing on boot up. Then we made some of his icons play her songs too when clicked.
After that we edited his registry so he could not change any of it.
He thought it was hilarious!