[ОлегА]

#menu .button_fixed { color:red; background:blue; }
#menu .button_no_fixed { color:fff; }

<div id="menu">  
 <ul>  
 <li><a href="#"><span class="button_no_fixed"><b>Главная</b></span></a></li> 
 <li><a href="#"><span class="button_no_fixed"><b>Форум</b></span></a></li> 
 <li><a href="#"><span class="button_no_fixed"><b>Галерея</b></span></a></li> 
 <li><a href="#"><span class="button_no_fixed"><b>ClanWars</b></span></a></li> 
 <li><a href="#"><span class="button_no_fixed"><b>Фильмы</b></span></a></li> 
 </ul> 
 </div>

$('#menu .button_no_fixed').click(function(){

   if( $('#menu .button_fixed').length ){
     $('#menu .button_fixed').removeClass('button_fixed').addClass('button_no_fixed')
   }

   $(this).removeClass('button_no_fixed').addClass('button_fixed')
})