只掉一次的窗口

下面为效果显示

这是只掉一次的窗口

脚本说明:
把如下代码加入<body>区域中
<script language="JavaScript1.2">
var ie=document.all
var dom=document.getElementById
var ns4=document.layers

var bouncelimit=32 //(must be divisible by 8)
var curtop
var direction="up"
var boxheight=''

function initbox(){
if (!dom&&!ie&&!ns4)
return
crossobj=(dom)?document.getElementById("dropin").style : ie? document.all.dropin : document.dropin
scroll_top=(ie)? document.body.scrollTop : window.pageYOffset
crossobj.top=scroll_top-250
crossobj.visibility=(dom||ie)? "visible" : "show"
dropstart=setInterval("dropin()",50)
}

function dropin(){
scroll_top=(ie)? document.body.scrollTop : window.pageYOffset
if (parseInt(crossobj.top)<100+scroll_top)
crossobj.top=parseInt(crossobj.top)+40
else{
clearInterval(dropstart)
bouncestart=setInterval("bouncein()",50)
}
}

function bouncein(){
crossobj.top=parseInt(crossobj.top)-bouncelimit
if (bouncelimit<0)
bouncelimit+=8
bouncelimit=bouncelimit*-1
if (bouncelimit==0){
clearInterval(bouncestart)
}
}

function dismissbox(){
if (window.bouncestart) clearInterval(bouncestart)
crossobj.visibility="hidden"
}

function dropornot(){
if (get_cookie("droppedin")==""){
window.onload=initbox
document.cookie="droppedin=yes"
}
}
dropornot()
</script>
<div id="dropin" style="position:absolute;visibility:hidden;left:250;top:100;width:500;height:300;background-color:#F5F5F5">

<div align="right"><a href="javascript:dismissbox()">[关闭窗口] </a></div>
这是只掉一次的窗口
</div>