1. Get to their computer, take a screen shot of their desktop. save it somewhere you can find it.
2. Then hide all their icons
3. make your new screen shot their desktop photo.
This will make it look like nothing's changed, but the icons won't work.
Also, my IT son has a program that turns your desktop 90 degrees.
My husband pulled this one on one of his co-workers several years ago. There's a program out there that makes it seem like you've gotten into the NAS computers (National Armed Services). It asks for a password three times. Doesn't matter what you put in, the third time it accepts it. Then it takes you through a series of questions that end up launching a SCUD warhead at Canada. It's very simple, no elaborate websites or anything. For some reason, that made it more believable.
Oh, and it takes about 6 large phone books to fill a normal sized dorm room chest high, if you rip each page out and crumple it.