First the $WINDOWS.~BT the folder is associated with the Windows 10 and Windows 8 upgrade process.
The advice is: Do not delete it.
The folder may use gigabytes of disk space.
If you really want to delete it then you can try to use Disk Cleanup tool.
This included with whatever version of Windows you’re using but click right on C:\ then select Properties.
On Disk Cleanup tool and click Clean Up System Files.
- Previous Windows installation(s) on Windows 10;
- Temporary Windows installation files on Windows 7 and 8;
Press the OK button to remove the files.
If don’t see one of this selection then don’t try to remove it.