Just type Control-Alt-Down on a Windows machine to flip the entire screen. This isn't a universal shortcut -- it depends on whether your graphic card supports it -- but it works on many modern laptops. Control-Alt-Up restores things to normal.
Is this cruel? Yes. But it can also serve to remind people of a more important keyboard shortcut: Windows-L to lock your system. Do that whenever you step away from the desk and your machine won't be tampered with.
This post is part of our Evil Week series at Lifehacker, where we look at the dark side of getting things done. Knowing evil means knowing how to beat it, so you can use your sinister powers for good. Want more? Check out our evil week tag page.