protected void Application_PreRequestHandlerExecute(Object sender, EventArgs e)
{
  string currentUrl = HttpContext.Current.Request.Path.ToLower();
  if(currentUrl.StartsWith("http://mydomain"))
  {
    Response.Status = "301 Moved Permanently";
    Response.AddHeader("Location", currentUrl.Replace("http://mydomain", "http://www.mydomain"));
    Response.End();
  }
}