I've done this a few times.
When your 'friend' is away from their pc or laptop, take a printscreen. Paste this into MS Paint and save onto the desktop (give it a generic name). Then Right-click the desktop and point at 'Arrange icons by' and untick 'Show Desktop Icons'. Then set the background of your desktop to the printscreen pic.
You could also right-click the taskbar and unlock it, then shrink it out of view.
What follows is mass confusion with frantic clicking of icons when they don't really exist.
They can restart as many times as they like, and nothing will happen.. It's then just up to you, when you want to put them out of their misery. :)