给点提示，其他的自己写吧
LeftPosition = (screen.width) ? (screen.width-w)/2 : 0;
TopPosition = (screen.height) ? (screen.height-h)/2 : 0;div1.style.marginTop=TopPosition;
LeftPosition = (screen.width) ? (screen.width-w)/2 : 0;
TopPosition = (screen.height) ? (screen.height-h)/2 : 0;div1.style.marginTop=TopPosition;
<hrad>
<style>
div{position:absolute;width:100;height:0;
left:expression((document.body.clientWidth-this.offsetWidth)/2);
top:expression((document.body.clientHeight-this.offsetHeight)/2);}
</style>
<script type="text/javascript">
function Div_cl(){document.getElementById("div_cx").style.display="block";}
function Div_cll(){document.getElementById("div_cx").style.display="none";}
</script>
</head>
<body>
<input type="button" value="我要找找" onclick="Div_cl()"/>
<input type="button" value="我要关掉你" onclick="Div_cll()"/>
<a href="#" onclick="Div_cl()">我要去连接你</a>
<div style="display:none" id="div_cx"><table width="246" height="240" border="1">
<tr>
<td bordercolor="#FF6633" bgcolor="#FFFF00">
</div></td>
</tr>
</table></div>
</body>
</html>
小例子！！！！
var h=600;//div的高
var l=screen.width/2-w/2; //计算出来的居中的left
var t=screen.height/2-h/2; //计算出来的居中的top
var iNodeHeight= parseInt(document.getElementById("对象ID").style.height);
window.onscroll = function()
{
var iNowLeft = (parseInt(document.body.clientWidth)/2-iNodeWidth/2)+parseInt(document.body.scrollLeft);
var iNowTop = (parseInt(document.body.clientHeight)+/2-iNodeHeight/2)++parseInt(document.body.scrollHeight);
document.getElementById("对象ID").style.left = iNowLeft;
document.getElementById("对象ID").style.top = iNowTop;
}
var iNodeHeight= parseInt(document.getElementById("对象ID").style.height);
window.onscroll = function()
{
var iNowLeft = (parseInt(document.body.clientWidth)/2-iNodeWidth/2)+parseInt(document.body.scrollLeft);
var iNowTop = (parseInt(document.body.clientHeight)+/2-iNodeHeight/2)+parseInt(document.body.scrollHeight);
document.getElementById("对象ID").style.left = iNowLeft;
document.getElementById("对象ID").style.top = iNowTop;
}
var s = "网页可见区域宽："+ document.body.clientWidth;
s += "\r\n网页可见区域高："+ document.body.clientHeight;
s += "\r\n网页可见区域宽："+ document.body.offsetWeight +" (包括边线的宽)";
s += "\r\n网页可见区域高："+ document.body.offsetHeight +" (包括边线的宽)";
s += "\r\n网页正文全文宽："+ document.body.scrollWidth;
s += "\r\n网页正文全文高："+ document.body.scrollHeight;
s += "\r\n网页被卷去的高："+ document.body.scrollTop;
s += "\r\n网页被卷去的左："+ document.body.scrollLeft;
s += "\r\n网页正文部分上："+ window.screenTop;
s += "\r\n网页正文部分左："+ window.screenLeft;
s += "\r\n屏幕分辨率的高："+ window.screen.height;
s += "\r\n屏幕分辨率的宽："+ window.screen.width;
s += "\r\n屏幕可用工作区高度："+ window.screen.availHeight;
s += "\r\n屏幕可用工作区宽度："+ window.screen.availWidth;
alert(s);
</SCRIPT>
<body>
<style>
#warp{
position: absolute;
width:500px;
height:400px;
left:50%;
top:50%;
margin-left:-250px;
margin-top:-200px;
border: solid 1px red;
}
</style>
<body>
<div id="warp" style="display:none"></div>
<input type="button" value="Test" onclick="document.getElementById('warp').style.display='block'" />
</body>
</html>width:500px;
height:400px;
left:50%;
top:50%;
margin-left:-250px;
margin-top:-200px;
注意margin-left和margin-top分别是width和height的一半可以准确定位在页面中间
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
</head>
<script type="text/javascript">
window.onload=function(){
var test=document.getElementById("test");
var width=parseInt(test.style.width);
var height=parseInt(test.style.height);
test.style.top="50%";
test.style.left="50%";
test.style.marginLeft=(width/2*-1)+"px";
test.style.marginTop=(height/2*-1)+"px";
}
</script>
<body>
<div id="test" style="width:100px;height:50px; position:absolute; border:solid 1px green; top:0px; left:0px;">aaaaaaaaaaaaaaabbbbbbbbb<br />
ccccccccccccddddddddddddddddddddddd
</div>
</body>
</html>