区域滚动问题？请进...

Question

<table border="1" width="34%" height="1">
  <tr>
    <td width="20%" height="1">&nbsp;</td>
    <td width="39%" height="1">
    <div id=aa style="overflow: hidden; height: 100">
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
      dsfdsafd<br>
    </div>
    </td>
    <td width="41%" height="1">
    <img height="45" src="up.gif" width="67"a onMouseOver=ds(6) onMouseDown="dstop();ds(15)" onMouseUp="dstop();ds(6)" onMouseOut=dstop() title="点住不放可快速向上滚动！">
    <p>　</p>
    <p><img height="45" src="down.gif" width="67" onMouseOver=ds(-6) onMouseDown="dstop();ds(-15)" onMouseUp="dstop();ds(-6)"  onMouseOut=dstop() title="点住不放可快速向下滚动！" ></p>
    <p>　
    </td>
  </tr>
</table><script>
var Timer=null
function ds(s)
{aa.scrollTop+=s
 Timer=setTimeout("ds("+s+")",100)
}
function dstop()
{
  if(Timer!=null)clearTimeout(Timer);
  Timer=null
}
</script>