var isNav, isIE
if (parseInt(navigator.appVersion) >= 4){
    if (navigator.appName == 'Netscape'){
        isNav = true;
        layerRef = 'document.getElementById(';
        styleRef = ').style';
        anchorText = 'text';
    }else{
        if (navigator.appName == 'Nav'){
            isNav = true;
            layerRef = 'document.layers[';
            styleRef = ']';
            anchorText = 'innerText';
        }else{
            isIE = true;
            layerRef = 'document.all[';
            styleRef = '].style';
            anchorText = 'innerText';
        }
    }	
}

function cambia(ImageName,ImageFile){
	ImageName.src = ImageFile;
}
function visualizza(nome_layer,stato){
	eval(layerRef + '["' + nome_layer + '"]' + styleRef + '.visibility = "' + stato + '"' );
}
function check_login_fields(oggetto){
	if(oggetto.user_name.value=="")
	{
		alert("Inserire username");
		return false;
	}
	else if(oggetto.user_password.value=="")
	{
		alert("Inserire password");
		return false;
	}
	return true;
}