Hi,
You would definitely want to uncheck Use LDAP before switching to windows authentication.
Windows authentication cannot work over the internet. It can only work on a local network. Also if you are using a proxy server or a proxying firewall that could be a problem.
By using Forms authentication and LDAP you could login users over the internet with their windows credentials, but this requires that the web server be dual homed so it has a public facing network card for the internet and a private facing card to talk to the LDAP server. You would definitely want to use an ssl certificate since susers will be entering their windows credentials in the login form in this scenario.
Hope it helps,
Joe