Someone

 CREATE TABLE `grupos_genteya` (
  `id_grupo` int(6) unsigned NOT NULL auto_increment,
  `id_fundador` int(6) unsigned NOT NULL,
  `activo` enum('0','1') NOT NULL default '0',
  `tiene_foto` enum('0','1') NOT NULL default '0',
  `tiene_foto_top` enum('0','1') NOT NULL default '0',
  `foto` enum('','gif','jpe','jpeg','jpg') default NULL,
  `foto_top` enum('','gif','jpe','jpeg','jpg') default NULL,
  `nombre_grupo` varchar(20) default NULL,
  `nick_grupo` varchar(20) NOT NULL,
  `descripcion` varchar(150) default NULL,
  `numero_usuarios` int(4) unsigned NOT NULL default '0',
  `veces_ganador` int(3) NOT NULL default '0',
  `puntos_semanales` int(6) unsigned NOT NULL default '0',
  `puntos_totales` int(6) unsigned NOT NULL default '0',
  `fecha_creacion` int(10) unsigned NOT NULL default '0',
  PRIMARY KEY  (`id_grupo`),
  UNIQUE KEY `nombre_grupo` (`nombre_grupo`),
  KEY `id_fundador` (`id_fundador`),
  KEY `activo` (`activo`,`puntos_semanales`)
) ENGINE=MyISAM AUTO_INCREMENT=20 DEFAULT CHARSET=latin1 


mysql> explain SELECT * FROM `grupos_genteya`  ORDER BY `id_grupo` DESC;
+----+-------------+----------------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table          | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+----------------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | grupos_genteya | ALL  | NULL          | NULL | NULL    | NULL |   19 | Using filesort |
+----+-------------+----------------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)