The reason I got this idea in the first place was because of an April Fool's prank I wanted to play on someone this year but never got around to:
Step 1) Get onto their computer when they're not around and screen-capture their desktop. Most computers have a "Print Screen" button which freezes whatever's currently on their screen into a 1024x768 jpg image.
Step 2) Save this image onto their computer somewhere they won't find it, then set this image as their desktop background.
Step 3) They won't notice the difference at first, because everything will look like it did before. That is, until they start trying to move icons around on their desktop and find that there's a non-movable duplicate-image stuck beneath it (part of the background), that can't be altered or deleted no matter how much they try. Watch their head explode as they try to figure out what the hell's wrong with their computer.
This was opposed to my alternate Fool's Day plan:
Step 1) Throw their computer into a lake.
It's a little less subtle, but I'm sure anyone watching would have to admit it would be a pretty good prank.