function ventana(url,nombre,ancho,alto,scroll)
{
 if (scroll==undefined)
 {
           scroll=1;
 } 
    var ventana;
    var top = (screen.height/2) - (alto/2);
    var left = (screen.width/2) - (ancho/2);
   ventana=window.open(url,nombre,"width="+ancho+",height="+alto+",resizable=0,toolbar=no,menubar=no,scrollbars="+scroll+",status=0,top=" + top +",left=" + left );
    
}