var ukbnbpopup;
function PopupWindow(url, windowname, width, height, scroll)
{
   if(ukbnbpopup != null && !ukbnbpopup.closed)
      ukbnbpopup.close();
   ukbnbpopup = null;

   var left = parseInt( ( screen.width - width ) /2 );
   var top = parseInt( ( screen.height - height ) /2 );
   ukbnbpopup = open(url, windowname,"scrollbars=" + scroll + ",resizable=no,status=no,height=" +height + ",width=" + width + ",left=" + left + ",top=" + top);
}