var loadEvent = function () {
  if ( $('topics') ) {
    var rand = Math.random() * 30;
    new Ajax.Updater(
      'topics',
      'catalog/ajax/topics.html',
      {
        method     : 'get',
        parameters : rand
      }
    );
  }
}

Event.observe( window, 'load', loadEvent, false );