Another thing to check - be it a long shot given it's a laptop PC:

Check the System Volume Information folder.
By default, only "System" has privileges to that folder, so space occupied by stuff in that folder won't show up in any disk space counts (including TreeSize)

I once had this same problem with a server, and found that some huge files were left in there by Back Up Exec when BUE was angry.
To fix it, I gave Administrator permissions to the folder, then I could delete the crap.

EDIT - Don't ramdomly delete stuff from that folder with out researching first - just remembered that I purged it by stopping/starting a specific service, not deleting files - let me know if you need more info - kinda a long shot.