Goating

Geek Dictionary:
Goating - When a user doesn't lock his computer before walking away, and allowing for others to pull pranks on him.

Some cool/classic pranks:
1) Replacing the desktop with a screenshot of the desktop, and hiding all the visible items on it.
2) Switching the mouse from right to left handed.
3) Using the video driver settings to rotate the display left, right, or upside down.
4) Switching the keyboard layout from QWERTY to Dvorak (or vice-versa).
5) Installing the bluescreen of death screensaver.
6) Installing a fake clippy that randomly pops up and says something annoying or insulting. http://www.rjlsoftware.com/software/entertainment/clippy/default.shtml or other software from http://www.rjlsoftware.com/software/entertainment/

Also something fun to do when bored working.