var fenster;

function xxx(url){
  
  	var top, left, width=720, height=505;

  	top  = Math.round((screen.height - height)/2);
  	left = Math.round((screen.width - width)/2);

  	if(fenster){
   		fenster.close();
  	}

  	fenster=window.open(url,'Meldung','width='+width+',height='+height+',left='+left+',top='+top+', scrollbars=yes, resizable=yes');
  	fenster.focus();

}