All my files appear gone, but I didn't delete anything and the capacity is still accounted for

I'm looking to update my old 2007 Macbook from Mac OS 10.5 to 10.6, but I've run into a problem..

Before I've even got to copying the .iso file I have (file in on my Macbook Pro at the moment) onto the computer I've gone to delete all of my music and other data so I have room as there was currently only about 400MB available on the hard drive. If I go into any folder, all my data appears to be gone.. Documents is empty, Movies is empty, Pictures is empty, Music has the iTunes folder but otherwise empty. I never deleted any of this stuff, and the capacity of the drive according to Finder is just as I'd expect it to be. My desktop picture is the same so that file at least is still there...somewhere!

Can anyone shed some light onto how I can make these files visible? They're obviously still there, just not accessible..