This is the javascript that makes popup window at the center of the screen

<script type="text/javascript">
	function popUp(URL) {
	day = new Date();
	id = day.getTime();
	var w = 480, h = 340;
	   w = screen.availWidth;
	   h = screen.availHeight;
	var popW = 400, popH = 170;
	var leftPos = (w-popW)/2, topPos = (h-popH)/2;
	eval("page" + id + " = window.open(URL, '" + id + "', 'toolbar=0,scrollbars=1,location=1,statusbar=0,menubar=0,resizable=1,width=" + popW + ",height=" + popH + ",left=" + leftPos + ",top=" + topPos + "');");
	}
</script>