Forms authentication not working

Hi everone,
I am developing a small website in vs-2005.
Here I am using forms authentication and I have used an access database. I have put the db in App_Data folder and declared the connectionstring in web.config. The problem I am facing is that when I request any page without logging in the browser displays the requested page. At the bottom of the page there is a script error when I check the details it says Sys is not defined. Has this something to do with this issue??
In web.config file I have added the following lines in the <system.web></system.web> section.
         <authentication mode="Forms">
                <forms name=".ASPXFORMSDEMO" loginUrl="Default.aspx" protection="All" path="~/" timeout="30"/>
             ...