A hacker with a malicious sense of humor might try to plant a little program designed to be wickedly annoying on your computer. For example, it might make your CD drive tray keep popping open, or the file you're working on keep minimizing. Or it might cause an audio file to play a scream at full volume in the middle of the night. None of these is much fun when you're trying to get sleep or get work done ... but at least they don't do any lasting damage.