赋什么值能将div总是显示在屏幕的中央

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<html>
<head>
<script language="javascript">
function switchSpan(spanId){
 
if(document.all[spanId].style.visibility=="visible")
document.all[spanId].style.visibility="hidden";
else{
document.all[spanId].style.visibility="visible";
document.all[spanId].style.left=document.body.scrollLeft+document.body.offsetWidth/2-315;
document.all[spanId].style.top=document.body.scrollTop+document.body.offsetHeight/2-76;
}
}
</script>
<script language="JavaScript">
var dragapproved=false
var z,x,y
function move(){
if (event.button==1&&dragapproved){
z.style.pixelLeft=temp1+event.clientX-x
z.style.pixelTop=temp2+event.clientY-y
return false
}}
function drags(){
if (!document.all)
return
if (event.srcElement.className=="drag"){
dragapproved=true
z=event.srcElement
temp1=z.style.pixelLeft
temp2=z.style.pixelTop 
x=event.clientX
y=event.clientY
document.onmousemove=move
}}
document.onmousedown=drags
document.onmouseup=new Function("dragapproved=false")
</script>
<style> <!--.drag{
position:relative;
cursor:hand;
background-color: #3399CC;
0Avisibility: hidden;
width: 315px;
height: 76px;
border: 1pt solid #000000;
0A}--></style>
</head>
<body bgcolor="#ffffff" text="#000000">
<div title="移动窗口" id="post" class="drag" name="post">发贴<a href='javascript:switchSpan("post")'>关闭<img alt="关闭发贴窗口"></a>
<table style="cursor:default" width="100" height="100" border="0" cellpadding="0" cellspacing="0" bgcolor="#eeeeee">
<tr>
<td>
<input type="text">
<br>
<input type="text">
<br>
<input type="button" value="button"></td>
</tr>
</table>
</div>
<input type="button" value="发贴" onclick='switchSpan("post")'> f f f f fff f ff 
f f f ff ff<p></p>
f<p></p>
f f ff ff<p></p>
f ff<p></p>
f<p></p>
f<p></p>
f<p></p>
f<p></p>
f f<p></p>
ff<p></p>
f<p></p>
f<p></p>
f<p></p>
f<p></p>
f f ff ff<p></p>
f<p></p>
f<p></p>
f<p></p>
f<p></p>
f ff<p></p>
f f ff ff<p></p>
<input type="button" value="发贴" onclick='switchSpan("post")' ID="Button1" NAME="Button1">
f ff<p></p>
f
</body>
</html>