It would be nice if Serv-U had an option to logon to HTTP/HTTPS using the current windows authentication credentials.
Seems like a good option to add under 'Limits & Settings' and control it by server or per domain.
Alternatively, you could add a checkbox below the username/password prompt to "Use Windows Credentials" which would gray out the fields, and then the user could click "Login." The browser could set a cookie to remember the setting.
Windows Authentication can help make it easier and more intuitive for company employees when adopting new systems.