Okay, if you read fanfare you should be relativly oriented to this prank. If not here is an overview:
Look on the back of your computer tower; you should see slits or holes. This is where the fan is. Without the fan the computer will overheat.
Go to the victim's computer with a pocket knife, some computers will have wires exposed (ie a tiger pc will have two, a black and red.) cut them and the fan will not work, the computer will overheat and if they don't notice it will be ruined.
If you did this you probably noticed alot of wires behind the fan or next to it. If you are even more observant, you would have noticed that they led to the main power plug.
This prank is also simple to carry out. If you really hate the person cut them (be warned, that /is/ vandalism) if not, disconect them and wait untill they take the computer in to the shop to find out why it won't start.
The second is also along these lines. Just remove the switch thing behind the startup or power-on button.