function ventana(url, titulo, w, h) { 
izquierda = (screen.width) ? (screen.width-w)/2 : 0;
arriba = (screen.height) ? (screen.height-h)/2 : 0;
w=w+10;
h=h+10;
settings = 'height='+h+',width='+w+',top='+arriba+',left ='+izquierda+',scrollbars= no resizable= no, status=no,menubar=no';
window.open(url,titulo,settings);
 }