So I have left over the "Windows.old" file on my C drive, which is taking up 36.8GB of precious space. I know deleting this will prevent me from "going back to windows 7", the option in the settings.
I have a Macrium reflect image of my C drive that I made right before I upgraded. Could I delete the .old folder, and later downgrade with the reflect image if I really wanted to? I'm not likely to go back to windows 7 unless I discover a HUGE problem.
I have also the "$Windows.~BT" and "$Windows.~WS" folders taking up 3.6GB and 5.6GB of space, respectively.
These are from Windows Update, correct? All updates have been installed according to the updater. These appeared before the upgrade. I should also note I executed the upgrade manually through the tool Microsoft supplied.

Any other space I could save?