function popup(url, w, h)
{
   var xPosition = ((document.width-w)/2);
   var yPosition = ((document.height-h)/2);
   window.open("" + url + "", "contentwin","width=" + w + ",height=" + h + ",scrollbars=no,status=no,resizable=no,screenX=" + xPosition + ",screenY=" + yPosition + "");
}