Want to play prank with your Geek friend then you can try using the Windows hide desktop icon feature. The following explains how to make your Windows Vista desktop completely useless using the hide desktop icon and also by enabling auto hide the taskbar option. This is a old trick that was done for Windows XP and this has been modified for Windows Vista. And with a little change, the same might work for a Windows 7 system too.
Click on the Show Desktop shortcut and take a screenshot of your desktop along with your taskbar (using Print Screen on Your keyboard)
Save the image to file to a location other than your desktop.
Right click on your desktop and select Personalize option from the menu list.
In the Personalization window, click on the Desktop Background link. This would display the following Desktop Background section.
Now choose the screenshot of the desktop image and set that as the desktop background.
Again navigate to your Windows Desktop then right click and unmark the Show Desktop Icons available as part of the View menu.
To add flavour to this fun, you can enable auto hide the taskbar and unmark keep the taskbar on top of other windows using Windows Taskbar Properties.
What this would mean is that you have got a a desktop that cannot be clicked at all and by making it completely unusable. Now you can ask your Geek friend to fix this problem for you!.