function getad (size,ad_div,prefix) {
	var myAjax = new Ajax.Request(prefix+'ads/getad.php',
	  {method:'get',
	  parameters:'size='+size+'&prefix='+prefix,
		  onSuccess: function(transport){
		 var result = transport.responseText.evalJSON(true);
		  if (result.success) {
			  $(ad_div).innerHTML=result.ad.inner_html
			 return true;
		  }
		}
	  });
}