I used to run the helpdesk for Citrix System internal. We had this policy whereby you were supposed to lock your workstation when you got up from your desk. Sarbanes and Data Security issues ya know... At any rate, one of the neat gags was against someone who would walk away from their cube and forget to lock their workstation:
#1: Take a screen shot of the person's desktop with all windows minimized using CTRL + PRTN SCRN
#2: Open Windows Paint
#3: Paste the screen shot into paint and save it as a JPG
#4: Highlight all the icons on the desktop
#5: Right Click and choose "Properties"
#6: Set "Hidden" as an Attribute and click OK. All Icons on the desktop should then disappear.
#7: Adjust the Start Bar at the bottom of the screen down to be completely minimized
#8: Right click on the desktop and choose properties
#9: Remember that JPG we saved in Step 3? Set it to be the wallpaper.
#10: Lock the workstation
This got really freakin incredible one day when one of my more challenged folks called me over and said he had rebooted his machine several times, but after every single login attempt, the machine would freeze at the desktop. I told him to reinstall the OS.
I love my job sometimes...