Data missing from directory listings

FullDisk scans all of the files and directories on a hard disk (including
hidden and system files) and calculates totals.
I ran FullDisk (in admin mode) on my 300GB hard disk, and got these results:
Used Space: 104,392,954,169
Free Space: 116,398,723,072

In "Local Disk (C Properties" as reported by Vista, I have:
Used Space: 183,689,109,504
Free Space: 116,398,657,536

I never had any such huge discrepancies when using FullDisk on w2k.

In a command prompt (in admin mode) I ran "vssadmin list shadowstorage" and
got:
Used Shadow Copy Storage space: 35.579 GB
Allocated Shadow Copy Storage space: 37.399 GB
Maximum Shadow Copy Storage space: 41.922 GB

So on Vista, even taking the shadowstorage into account, about 38GB is
unaccounted for in FullDisk.
And since FullDisk isn't accounting for it, that means that it's not showing
up in directory listings. Where is this mystery 38GB of data?