metti nell'head
codice:
<script>
function PopUpCentroBrowser(pagina,nome,w,h){ 
	if(document.all){
		var ax = window.screenLeft;
		var ay = window.screenTop;
		var aw = window.document.body.offsetWidth;
		var ah = window.document.body.offsetHeight;
	}
	else{
		var ax = window.screenX;
		var ay = window.screenY;
		var aw = window.outerWidth;
		var ah = window.outerHeight;
	}
	x=ax+Math.round((aw-w)/2);
	y=ay+Math.round((ah-h)/2);
	var tl='top='+y+',left='+x+',';
	window.open(pagina,nome,tl+'toolbar=no,status=no,scrollbars=no,width='+w+',height='+h);
}
</script>
e all'onsubmit
codice:
onsubmit="PopUpCentroBrowser('','popup',320,240);