Here you all go. This one will nicely mess with someone's PC.
Get on a computer. Schools, hated person, friend you want to mess with, etc. Go to search and look up "RegEdit" without the quotes. An EXE program will be in the list, click on it. When you do you will be brought to a window that looks similar to the Windows Explorer. Welcome to the Registry. If you erase files or sections in this, you can do some damage. The registry contains links, and coding to all the programs, symbols, statistics, etc of the computer. If you erase sections such as "Software", "Drivers", Hardware". You can make the computer not recognize that these units are here. They will be corrupted, they won't work.
This is a nasty prank. Do only to those who deserve it. CANNOT UNDO THE CHANGES -- WILL CAUSE DAMAGE!