If you really want to screw with somebody without installing anything, just enable the Dvorak alternate keyboard layout that even most geeks aren’t familiar with. When they type, everything will go haywire.
Head into Control Panel –> Region and Language –> Keyboards –> Change keyboards, then click the Add button and pick the Dvorak layout (or any other random layout you want). Once you’ve done that, use the drop-down above to set the default.
You’ll probably want to flip over to the Language Bar tab and set that to hidden as well, so they won’t be able to figure it out easily.