The dilbert website just tried to install somehting on my work PC (only time I've ever been glad to not have administrator privileges):|
If this is for basic tech support, I would say install kazaa or maybe imesh, and browse a few 'questionable' sites. You want to simulate real problems.
Try hiding some documents in weird places too, then have them find 'that file I was working on' which you don't know the name of. This is especially nice if the computer crashes and you need to track down the office auto-backup version (though that hasn't been as necessary since Office 97/windows 98 stopped being a standard config).