Here's a prank you can play on someone that I just thought of, it's quite silly and immature but fun for the few minutes it lasts. (Or if you pick a really dumb victim, it may go on for ages, in which case you may split your sides open with laughter). I actually just did it to myself by accident, and was almost fooled for a second lol.

1) Take a screenshot of your victims desktop, paste it and save.

2) Then, hide all icons on his/her desktop, and apply the screeny as the background pic.

3) Sit back and grin as they become confused, then angry, as they click away at the icons like a possessed person wondering why nothings happening!

At this point, you could make things even more interesting by saying you think you downloaded a virus or something. Then they shout at you, and maybe even turn a shade of green. They shouldn't figure it out unless they go into display settings (they may not even realise then lol)

It's the work of a genius