I do remember one time at work when they were messing about with the domains, and had to switch our user accounts over, and somehow managed to create two user accounts, duplicating the common files, including the desktop. At the time I had a number of huge files on my desktop, which resulted in a couple of gig of 'lost' space till I tracked it down (with the above method). If the guest account was created as a duplicate, it might be the cause... or at least one cause, since I doubt he's been keeping 30 Gb of files on his desktop :P