哎~~~纠结死了的终于解决的i6的fixed属性

css部分： 

* html .backtop /* IE6 底部固定 */{position:absolute;bottom:auto;top:expression(eval(document.documentElement.scrollTop+document.documentElement.clientHeight-this.offsetHeight-(parseInt(this.currentStyle.marginTop,10)||0)-(parseInt(this.currentStyle.marginBottom,10)||0)));}
.backtop{bottom:300px;
height: 120px;
left: 0;
margin-left:10px;
position: fixed;
visibility: visible;
120px;}

.gotop{
background: url("http://www.wanbu.com.cn/images/v7/tsw_ad.gif") no-repeat scroll 0 0 transparent;
display: block;
height: 120px;
120px;}

 html部分：

<div id="ad" class="backtop">

<DIV onclick="hidead();" style="FONT-SIZE: 9pt; CURSOR: hand; "align=right>关闭×</DIV>
<a href="http://www.wanbu.com.cn/changzheng.php?aid=203" target="_blank" class="gotop"></a>
</div>


<script type="text/javascript">
function hidead()
{document.getElementById("ad").style.display="none";}