just putting a pic of your desktop as the background is fairly tame.
if your computer video drivers have the option to rotate the display then flipping the desktop pic by 180 and placing it as the backdrop then rotate the display through 180 as well really confuses people.
playing about with Autocomplete on word is always a good one to wind people up too (set it so it changes their name to something else every time it's typed)
Shutdown everytime you startup is easy to solve, just boot into safe mode and clear whatever has been put in the startup folder (there are other ways to do the shutdown trick, but thats the normal one)
oh and a 15minute startup/shutdown sound is pretty annoying as well