watch them die inside as nothing works, not even a restart.
Optional step:
put your wireless mouse receiver in their computer and subtly click occasionally, or move when they move. Has to be subtle enough for them to question their sanity for a few hours.
That's a great addition. We used to screenshot it with an application open so they would keep trying to close the application. I had a coworker restart their computer several times before he realized what was going on.