I never had to use Zadig in Windows 10 64bit to use DFUseDemo. This is probably a problem by you. Put your Hardware Device Manager window on and examinate following. Try uninstalling Zadig (actually a USB driver) until it no longer appears in Hardware Devive Manager window. This may have to be done multiple times. In the View menu, put the option "show hidden devices", you will see them grayed out in the list, and see if there is an earlier installation of Zadig (Zadig itself is not visible, but 'USB lib device' or something similar - i have not the exact names, but don't delete the 'Universal Serial-Bus Controllers'!) delete or uninstall all gray drivers until they are gone. The Wiki Files contains the manual of DFUseDemo and its installation. Follow it carefully. I test it again here, and you don't need Zadig. If you connect the Nano, with the boot button pressed, a "STM Device in DFU Mode" will first appear in Hardware Device Manager (with a yellow triangle). If you wait 20 seconds, the driver is automatically loaded in Windows, and the "STM Device in DFU Mode" is added to the list "Universal Serial-Bus Controllers". If you turn the Nano off, "STM Device in DFU Mode" will remain in the list, but then turn gray.
on1bes