Well, all good suggestions... does anyone know the combo to invert the screen (up to down)? I forgot it....
This is a tutorial on how to take control of another computer. Useful for some laughs! Also, for the less tech savvy, just switch the "M" and "N" keycaps on the keyboard, they'll never notice! Another is to wait for someone to walk away from their PC. Click Start -> Settings -> Control Panel -> Mouse. Select the "Pointers" tab and change the first cursor (Normal Select) from an arrow to the same thing they use for the "Busy" cursor. See how long they will stare at their screen waiting for the computer to finish what it's doing. Also, take a screenshot of the prankee's desktop (hit the print scr key) and paste the image into MS Paint. Hide all of the desktop icons and set the screenshot as their background!
make a folder on the desktop saying "Horse Porn" or something of that Genre. Then take a screenshot of the desktop and have that screenshot as the desktop. Remove the folder and the picture of it will still be there. This works really well for those that are very un-educated in the art of computers.
I have a site which have thousands of computer prank which will surprised your friends, it’s not virus but funny things which I usually put it on my known friends. You can see here for more computer pranks.