在做漂浮广告时候，怎样把鼠标放到那里漂浮广告就停止！

<div id="ad" style="position:absolute">
<a href="http://xu-ni.net" target="_blank">
<img src="http://xu-ni.net/bbs/skins/copyright.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 <