Asp – Windows authentication. User gets prompted – ASP.NET

I have the following settings:

Web.config

Authentication Mode = "Windows"

Virtual Directory
-> Enable anonymous access – UNCHECKED
-> Integrated Windows authentication – CHECKED

When a user goes to the default page on the intranet, he gets prompted to enter the user name and password even though he has already been authenticated

**UPDATE:
To clarify what I am doing
– We have an application that is set to
windows authentication = forms
– There was a request that for certain servers in the intranet, we would want to bypass the login.aspx page
– All that the login.aspx page does is, grabs the windows username, checks the database, gets the forms username and password and lets the user log in
– I am using User.Identity.Name to get the windows user name to do a mapping

**