1. Implement Forms Authentication and Authorization in Web.config
<system.web>
<authentication mode="Forms"/>
</system.web>
<location path="<filename or relative path of the folder>"> <!--e.g. PublishedFiles\Category1, or PublishedFiles\Category1\file1.htm-->
<system.web>
<authorization>
<deny users="?"/>
<allow users="*"/>
</authorization>
</system.web>
</location>
2. Ensure that there is an entry for .htm/.html in IIS Configuration.
Forms Authetication will not work if you dont have the entry for corresponding file extension in IIS configuration.