So Amit pointed me to this post on Coding Horror (I didn't get time to read any of the feeds for the last 2 days!!). It's about colleagues playing pranks with folks who leave their computer un-locked. I myself have done couple of these stupid but amazingly successful ones like taking screen shot of the desktop and then making it the wallpaper. Then you hide all visible things on the desktop so that the user on retuning goes on clicking on them without any result.
However, these days at Microsoft the prank has changed. On returning you just get to see an email typed out, addressed to Bill Gates. I don't even want to get into the topic of what is typed. The effect is amazing and permanent. Win+L keys become etched in your reflexes.