Add the following key to your user.config file after replacing the value with the page url for the page you want people to use for logging in.
<add key="LoginPageRelativeUrl" value="/Secure/Login.aspx" />
After adding it to your user.config file, you will need to open the web.config, add a space to the bottom of the file and then save it. This will force IIS to reload your site and reread the user.config file.
HTH,
Joe D.