Nicolas Cage
Use this executable to prank your friends. Get it to execute on their computer somehow. Timer, script, whatever. That part is up to you. One easy way to do this is to place the program (or a shortcut to it) in the start menu -> Programs -> Startup folder.
Download it here: https://github.com/nylen/cage/raw/master/bin/Debug/cage.exe
All it does is show this image in a partially transparent window that allows click-through:
Press Ctrl+Alt+Shift+F12 to close the program (or kill it using Task Manager).
Go through the source code and compile it yourself if you're into that sort of thing.