20Gb of Hard Drive space is unnacounted for

Something is consuming 20 GB of hard drive space on my basically out of the box 60Gb iBook G4 with Tiger that I can't account for. The only thing that's been added is MS Office and about 1Gb of files. The hardware profiler claims 39Gb used of 56Gb, but if I Get-Info on the hard drive or files on the drive they never add up to more than about 19Gb used. I tried WhatSize and it also confirms only about 19Gb used on the whole drive. Any suggesting how I can find out what's taking up the other 20Gb? How much space does the factory installation of Tiger take up?
Thanks in advance for any hints you can provide.