So you have 'invisible' files that are about 3GB large; and you wonder why and how?
think about the pagefile.sys, hibernation file, system restore and other services. That will consume space. If you want to free up that space, try the Clean Up Wizard or whatever it is called on Windows. It will delete unnecessary things and give you more space.
Exactly WHY do you think your disk space is being reported incorrectly? (Your hard drive is not "memory" by the way). I am inclined to think that maybe you are mistaken about only using 6.5 gig of space on your drive.
Dear, that's because of virtual memory, right click on my computer go to the advanced tab, performance, choose settings, and again advanced tab, virtual memory, change, now set the disk space or turn off paging file mode to have your entire space back.
if the problem isn't yet solved .. don't let me know please