function check_fields() 
        {
            if (document.main_form.uid.value =="")
            {
                alert("Inserire USERNAME");
                document.main_form.uid.focus();
                return(false);
            }   
            else
            {
                window.location ="/login_" + document.main_form.uid.value + "/default.asp"
                document.main_form.uid.focus();
                return(false);
            }

            alert("USERNAME errato");
            document.main_form.uid.focus();
            return(false);
        }