The true size of the free space on your drive is 16,590,483,456 bytes, exactly as reported by the disk properties. The discrepancy is accounted for by files or folders that you don't have permission to access. The screenshot shows 2 of these, RRbackups, and System Volume Information - the page file also accounts for some of the missing space. It could well be that there are other directories at a lower level that also aren't reported by the file tree program that you ran because of access issues.
I'm still a little hesitant because my computer is still running great and I've never had a drive 85% full run well. (maybe all this Vista stuff we've read about really works).
edit: I deleted about 10 gb after it ran sluggish and after a day or two after that, my computer seems to running normally.
Any ideas on how to measure the size or view the hidden folders from windows? I really don't think it is necessary to have a 30 Gb backup ondisk where i've already made the recovery disks.
You'll need to log on as Administrator to see what's in those folders that you are not authorized to access. Don't suppose that your laptop is an IBM (or Lenovo) is it? Apparently they have a utility called Rescue and Recovery that creates an RRbackups folder that can be tens of Gigabytes in size. Here's a thread that tells you how to get rid of the RRbackups files.
Could also be worth running the Disck Cleanup option if you haven't yet done so - this might find other wasted space.