Cover their desk with paper cups. Staple them all together at the brim. Fill them with water.
Get a wireless mouse, plug the receiver into the back of their computer. Every once in a while move the mouse around.
Ctrl-alt-up arrow and ctrl-alt-down arrow will turn his screen upside down.
Tape down the switch-hook on their phone, then call them.
Change their mouse pointer to the hourglass.
left-al + left-shift + print screen turns on high contrast mode.
Mod his hosts file to block access to any website you want.
right-click your pal’s icon to Internet Explorer or some other commonly used program. Edit the properties and change the target to: “%windir%\system32\shutdown.exe -r -t 00″ Now, every time your buddy tries to run IE, his machine will mysteriously restart. You could put this in the start-up folder if you are really evil.
Swap the mouse buttons.