As I understood you want the login page to be accessible to everyone, and only after successful login be able to go to other pages...
For this you have to separate your site to folders
[root]
login.aspx
web.config
\site
home.aspx
web.config
In the outer web.config write this...
<configuration>
<system.web>
<authorization>
<allow users="*" />
</authorization>
</system.web>
</configuration>
In the inner web.config write this...
<configuration>
<system.web>
<authorization>
<deny users="?" />
</authorization>
</system.web>
</configuration>
The first says that anyone allowed to see pages in this folder, and the second says deny access to all not authorized person.
[If you permit a should add that in my opinion this way of login is a bit old-style. I was suggest you to use an other schema. The person visiting your site always seeing a home page with reduced content. Somewhere on this home page you have a corner that enables login. After the login the home page displays content more specific to the person just logged in...]