Remove All Desktop Icons
This is the oldest, yet still the best PC prank. You'd be surprised how many people haven't seen this trick, and even if they have, it's still a good laugh.
All you need is a minute or two alone with their PC. Close or minimise all open applications and take a screenshot of the desktop. Pressing the PrtScrn or Print Screen button (you might need to also hold Shift if it's a dual-function key) takes the screenshot.
You then need to paste it into any image editing program (MS Paint will do) and save it somewhere.
Next, clear all the desktop icons into a folder somewhere and set the taskbar at the bottom to auto-hide (right click on it and choose Properties, then click 'Auto-hide the taskbar'.
Next, clear all the desktop icons into a folder somewhere and set the taskbar at the bottom to auto-hide (right click on it and choose Properties, then click 'Auto-hide the taskbar'.
Now head to the Control Panel and set the image as the desktop background. For a quicker way, right-click on the desktop and choose Properties in XP, or Personalize in Windows Vista, 7 or 8.