This will make the untrained computer user think there's something wrong with their computer. It only works on windows. it will make the computer almost unuseable untill you turn it back on. I suggest making them get you a soda first.
Step 1: Task Manager
They must first leave their computer or you can ask if you can use it really fast. Once you have it click Control+Alt+Delete. this will get you to the task manager. On vista you must click after this "start task manager."
Step 2: Ending Explorer
Click the tab named "processes" and scroll down to find "explorer.exe" click on it then click "end task". Everything will be gone at that point the start menu the windows because that's all explorer is. Unless they know what they're doing their reaction most likely would be. What did you do?!
Step 3: Fun's Over
After you've had your fun and made them do numerous thingts for you. Go back to the task manager and click File+Run/New Task and type in explorer.exe then click enter. That should bring everything back to normal or restart the computer by holding down the on button.