Did you do the Anniversary upgrade? If so, be aware the upgrade creates a windows.old folder that holds all your old stuff should you need to revert back to your original OS. In this case the old version of Windows 10 prior to the Anniversary release.
With that, you can delete the window.old folder by doing a disk cleanup. When you bring up disk cleanup, click the Clean up system filesbox, and look for a box that says Previous Windows installation(s). Check that box and windows should remove the Windows.old folder.
Be aware once you delete this folder you will not be able to revert back to your old OS. On the flip side unless your system is really broken, you shouldn't need these files.
Here's a tutorial on how to delete the windows.old folder - How to Delete Windows.old Folder in Windows 10
Good luck. BTW my windows.old folder was some 20.4 gig large.