var popupWindow = null;
function popup(link, w, h)
{
   if(popupWindow != null)
      popupWindow.close();
   var x = (screen.availWidth  - w) / 2;
   var y = (screen.availHeight - h) / 2;   
   var propertyString = "scrollbars=yes";
   propertyString += ",width=" + w;
   propertyString += ",height=" + h;
   propertyString += ",left=" + x;
   propertyString += ",top=" + y;
   propertyString += ",screenX=" + x;
   propertyString += ",screenY=" + y;
   popupWindow = window.open(link, "", propertyString);
}