Was that even real? He was ducking down well before the sounds came on.
Now a real piss off pranks would be... Screen cap a persons desktop...(make sure the pointer is off screen) go to paint open the screen cap, save it. Then go back to the desktop, right click, select Arrange icons. Then click on the show icons selection, making all icons disappear. Then right click on desktop again, select properties, then desktop, then browse for the pic you just saved. Use that pic as the new background image for the desktop. when your "friend" returns, watch the hilarity ensue as he/she tries to click on an icon to do something. Dont forget to hide the Taskbar too.