showHideLayers高手帮忙看看！

<script language="javascript">function showhide()
{
  var o=document.getElementById("aa");
  if(o.style.display=="none")
     o.style.display="block"; 
  else o.style.display="none";
}
</script>
<a herf="#" onmouseover="showhide()" onmouseout="showhide()">asdfasdf</a>
<iframe id="aa" src="http://www.21w.com/xlh_xlh/right.php" width="171" height="100%" frameborder="no" border="0" marginwidth="0" marginheight="0" scrolling="auto" style="display:none"></iframe>