Server Unavailable 503 error

I am trying to get an new INTRANET site up but I keep getting the following error "SERVER UNAVAILABLE" message.  On my connection pool under the identity tab I have selected Configurable and enter in a guest account that is defined to IIS_WPG group.  The password is correct and I have the following code in my  web.config file (as a test)
 <authentication mode="Windows" />
      <identity impersonate="true"
                userName="Guest"
                password="Guest"
   />
I have tried this without putting a username or passaword and I still get the same result.  
If I change the Identity  within IIS to predefined then the site asks for a user id and password which I have to use the Administraor userid and password and the site works fine.  I am trying to have the site work without having the user enter an credentials.
I am sure there is a simple answer but I don't have a clue to what is causing this issue.