I used to install BSOD screensavers with the option to list the particular comp's details.
And even further back in the day, I'd rename the name of the hard drive to LOSING DATA so when they'd do a /dir, they'd get 'Drive C: is LOSING DATA'.
I'm evil.
Even better is screenshotting your coworker's current work screen. And then make it his desktop background. He'll think the comp is frozen.