U.Helper

$('a#poisk').click(function(){
  if($('div#fullpoisk').is(:hidden)){
  $('div#fullpoisk').slideDown('normal');
  $('div#fullpoisk').load('/forum/0-0-0-6/ ');
  }else{
  $('div#fullpoisk').slideUp('fast');
  };
});

<a href="javascript://" id="poisk">Поиск</a>
<div id="fullpoisk"></div>