well we always messed with computer iliterate teachers by;
1) "prt scrn" and capture a picture of their desktop (icons and all).
2) set it as wallpaper.
3) then move all the icons as far off the screen as you can.
this might be useless against the more tech savy faculty, but is still amuzing to watch people's reaction when they can't get anything to respond. also when the icons are not working the start bar is the first thing they go for. hide it and move it to a corner of the screen.
*btw: fixing this is as easy as right clicking and re-aranging the icons. they might not think its funny if you wait till they re-format their hard drive.