Centrering af Layer

Hvordan får man et Layer til at centrere på siden:
<head>
<script language="JavaScript1.2">

// Drop-in content box- By Dynamic Drive
// For full source code and more DHTML scripts, visit http://www.dynamicdrive.com
// This credit MUST stay intact for use

var ie=document.all
var dom=document.getElementById
var ns4=document.layers

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

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)+215
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 redo(){
bouncelimit=32
direction="up"
initbox()
}

window.onload=initbox

</script>
</head>
<body>
<div id="dropin" style="position:absolute;visibility:hidden;left:250;width:457px;height:211px;background-color:#F5F5F5">
</body>