When they're away from their computer:

1. Take a screenshot of their desktop

2. Create a new folder somewhere on their hard drive and move all of their icons/files from the desktop there

3. Auto-hide their task bar (right click on the task bar and go to properties)

4. Set the screenshot from Step 1 as their desktop background

5. Watch them click away at the image of their icons and start menu when they come back to their desk! This can literally provide hours of entertainment, especially if they can't figure it out and they call the IT department.