wheat,
you just reminded me of an April fools I did one year,
I took a screenshot of a dialog box that said:
Reformat System Drive?
Okay
no cancel button,
then I made it the guy's desktop and hid his start menu bar, but there was a start menu bar in the desktop picture, so it looked like his screen was frozen because none of the desktop shortcuts worked, it would be that way if he restarted, and he wasted an hour trying to fix it until we all couldn't take it anymore.