<system.web>
<authentication mode="Forms" >
<forms
name="MyApp"
path="/"
loginUrl="Logon.aspx"
protection="All"
timeout="20">
</forms>
</authentication>
</system.web>

You will need to add after the </system.web> something like this:-

<location path="Public">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>

This will allow anyone (authorised or not authorised) to access the pages in the "Public" directory.