Remote access software enables you to control one PC from another, Apps such as TightVNC can be installed without leaving any obvious sign that they’re running (unless your victim’s using Windows Vista: its beefed-up security makes such mischief considerably harder to get away with).
With VNC installed on your machine and your target’s, you can take control of their PC from anywhere with an internet connection. Make sure your firm’s IT policies don’t prohibit installing unauthorised software or fiddling with firewalls, though: any remote control software is a potential security risk and some companies won’t see the funny side.
You should also remove the software as soon as you’ve pulled your prank.
Or there’s Office Poltergeist, which is like VNC, but designed specifically for messing with your co-workers. Once you’ve installed the server on your victim’s PC and the controller on yours, you can open and close their CD drive, play silly sounds or pop up alert boxes on their screens.
Don’t visit the Google group that’s linked to from the Office Poltergeist website, though: it’s been defunct for ages and has been overrun by spammers of the ‘not safe for work’ variety.