This is a fun ploy; you jump on your co-workers computer and find a shortcut on their desktop to Word or something.  You right click on it and change the target to something more amusing.  A few things come to mind like

Shutdown –f –s –t 0
Which will shutdown the computer right after they click it.

Or try choosing any of the rundll32 command from http://www.dx21.com/SCRIPTING/INDEX.ASP a great site for reference when you are doing any scripting.  A couple of my favorite rundll32 commands are

RunDll32.exe user32.dll,LockWorkStation
which locks the workstation or

RunDll32.exe USER32.DLL,SwapMouseButton
which will swap their mouse buttons (effectively making their mouse left handed)

Use your imagination on this one, you can run any program on the computer, if you come up with a clever one make sure you post it to the comments.  Also if you have any office warfare ideas you think should be posted on the site send them to tips@officewarfare.net.