Help: Form Authentication vs Integrated Windows Authentication

I want to authenticate users using Active Directory and I am trying to figured out if I should be using Form Authentication or Windows Authentication.
Without modifying anything under Directory Security in IIS :
If I add the following code under web.config, a popup box appears with a username and password fields to Authenticate users
<authentication mode="Windows"/> <authorization> <deny users="?"/> </authorization> This works but does anyone know if this is secure? If SSL is installed, does this popup box encrypt information before sending it back to the server and does anyone know what a request/reponse process looks like in this scenario? Plus, how does this work, when LDAP information is not ...