function Overtura(theURL,winName)
{ 
  var izq=(screen.width-750)/1.4;

  var sup=(screen.height-430)/2;

  win='width=450,height=420,top='+sup+',left='+izq+',toolbar=no,location=no,resizable=no,status=no,scrollbars=0,directories=no';

  window.open(theURL,winName,win);
}