nel HEAD
script language="JavaScript">
<!--

function PopupCentrata() {
var w = 500;
var h = 220;
var pw = Math.floor((screen.width-w)/2);
var ph = Math.floor((screen.height-h)/2);
window.open("virtual.html","","width=" + w + ",height=" + h + ",top=" + ph + ",left=" + pw);
}
//-->
</script>
<script language="JavaScript">

il link
nomelink