It's good sense to lock your computer when you're away. My co-workers have a habit of punishing those who don't by changing their desktop background to a picture of David Hasselhoff (usually the one where he's naked holding a puppy), a practice known as "Hoffing". I decided to go one further and create a screensaver that displays mister Hasselhoff in all his glory.
This is the result. Extract the file on your victim's computer, right-click the .scr file and choose "Install". And here is the source code in case you're worried about malicious code (or just want to see how it works). I leaned heavily on the code from Frank McCown at http://www.harding.edu/fmccown/screensaver/screensaver.html.