function ventana(uri, ancho, alto){
	derecha=((screen.width-ancho)/2);
	arriba=((screen.height-alto)/2);
	string="toolbar=0,scrollbars=1,location=0,statusbar=0,menu bar=0,resizable=0,width="+ancho+",height="+alto+",left="+derecha+",top="+arriba+"";
	fin=window.open(uri,"",string);
}