Hi
Some of your hard drive is being use by your virtual memory.
On my computer it's about 9 gigabytes in system managed size on my C:\ drive and is set to 12 Gigs on my D:\ drive.
If you wan't to you can download and run defraggler, it will improve your system performance and show you a graph of what is on your hard drive and how much space it's using.
Defraggler - File and Disk Defragmentation - Free Download
It will take a while to run the first time but after that you can tell it to just defrag the fragmented files and it will do it in a few minutes.
I use it every other day and it can really make a noticeable difference in performance.
Mike