There's always the "mess with his computer desktop" thing. Get on his computer (assuming Windows), minimize everything, and hit print-screen. That captures the entire screen. Paste that into the paint program, and save it somewhere on the computer. Then remove all icons from his desktop so he has a blank screen. Then change his background wallpaper to the screen-capture you made. Voila, looks like his normal desktop, acts like his computer isn't working.