A really cool trick to play on one of your friends is to create a batch file, disguised as any program you want (such as Internet Explorer) that opens itself continually until the computer crashes.
To do this first open notepad and copy the code below.
Next save the document as joke.bat to a folder you know you're friend will not accidentally open (such as C:\windows). Now, right click your desktop and select New | Shortcut and type in the path of where you just saved the file in the text box. Click next and now enter the name of the file you want to disguise your batch file trap as (such as "Internet Explorer"), and click finish. After the shortcut is created, right click it and select properties. Next, click the change icon button and choose the icon you wish your trap to appear as. Finally delete the real original desktop icon so that only your trap icon is visible. Now when your friend goes to click on Internet explorer, my computer, or whatever you disguised your trap as, he/she will unknowingly open up a infinite number of windows which will after a few minutes cause your friends computer to crash.
Note: I've used this trick many times without any problems because after the computer is restarted the computer is fine, but don't blame me if there is any problems with this trick.