Hello,
I am not 100% sure and I cannot confirm it as I am on a Mac (I have asked support), the USB driver should be there, but not sure about the extension.
What I do remember is that the CC121 extension should appear in the control panel.
If you go to uninstall software it should be on the list.
Try uninstalling it from there, and then reinstall only the extension.
Also, have you tried running Cubase as administrator?
Best regards,
GN