A program that would be invisible in the backround and randomly flicker the screen white for a fraction of a second. It would also drop the refresh rate down to the lowest supported and move the icons around 1 pixel at a time on the screen to drive your vic out of his mind. Maybe also it would randomly fix everything for a couple days then start over.
Other ideas with this in mind:
Random reverse calibration on your mouse or slow the mouse for 2 seconds and then speed it up to highest for 2 seconds then back to slow etc...
Randomly show error messages that look like they mean something but actually mean nothing.
Have a randomly bouncing start button which runs away from you when you try to click it.
When the cursor has been sitting blinking for over 10 minutes, AND the screensaver has turned on, force the keyboard to type, "Somebody help me..."
Ok, yes, you may say it. I'm evil.