Conrolling Site Access Via Windows Authentication And Roles / Groups

Hi
I am trying to restrict access to an intranet site via Windows authentication. I would like to control access via active directory user groups, but my added AD groups don't appear to be recognised by .net. I started with the following test code in my app to determine if the group could be used...
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load        
Me.Label1.Text = User.Identity.Name        
Me.Label2.Text = User.IsInRole("Application Admin")        
Me.gvRoles.DataSource = Roles.GetRolesForUser        
Me.gvRoles.DataBind()        
If Not Roles.GetRolesFor ...