网页漂浮广告和网页错位的问题（10W火急）50分求解

[fyqr2000]

我在首页上作了一个漂浮广告的带码如下(可行动网上找的):
<div id="ad" style="position:absolute;visibility: visible;"><a href="http://www.flash8.net" target="_blank"><img src="http://www.flash8.net/images/logo.gif" border="0"></a></div>
<script>
var x = 50,y = 60
var xin = true, yin = true
var step = 1
var delay = 10
var obj=document.getElementById("ad")
function floatAD() {
var L=T=0
var R= document.body.clientWidth-obj.offsetWidth
var B = document.body.clientHeight-obj.offsetHeight
obj.style.left = x + document.body.scrollLeft
obj.style.top = y + document.body.scrollTop
x = x + step*(xin?1:-1)
if (x < L) { xin = true; x = L}
if (x > R){ xin = false; x = R}
y = y + step*(yin?1:-1)
if (y < T) { yin = true; y = T }
if (y > B) { yin = false; y = B }
}
var itl= setInterval("floatAD()", delay)
obj.onmouseover=function(){clearInterval(itl)}
obj.onmouseout=function(){itl=setInterval("floatAD()", delay)}
</script>
但我加在我的首页里面的时候这个漂浮广告是出来了。但别人在网页的底层漂浮了，本来要的效果是在网页最上层漂浮的。可现在就跑到最下层去了。好急哦。请各位大哥帮帮小弟