As I mentioned before, the installation added $Windows.~BT & $Windows.~WS folders plus a windows.old folder. Some of this is the installation media, some of it is a backup of your Windows 7/8.1 data. If you are certain everything is working fine and your data and apps have transferred over, you can free up some disk space (about 16 GB in my case) by removing these folders. This can be done via the Windows Disk Cleanup Tool which you will need to be logged in as an administrator to access the ‘Remove System Files’ feature.
SuperSite for Windows provides detailed step-by-step directions on how to cleanup your system after the Windows 10 update.
