On the C drive, there is a hidden folder called $Windows.~WS. My folder was 5.76 GB in size, containing 1,008 files and 103 folders. This folder contains one file, installx64.esd, that is over 2GB in size.
So, is it safe to delete? Well, I would first suggest you to read my blogpost about recovering lost disk space in Windows 10 after upgrading from a previous version of Windows. The blogpost outlines how you can remove unnecessary files after an upgrade.
If, after following these instructions, the folder is still there: read on.
In most cases, both $Windows.~BT and $Windows.~WS are indeed safe to delete. However, these folders may contain recovery files, in case you want to recover Windows (for example, when a failure happens). Click on either of these folders, then hold Shift and press Delete to permanently remove the folder(s).
If you get the message “You’ll need administrator permission to delete this folder”, click on continue.
This should add about 5GB free space to your C drive.