急，如何用JavaScript让div居中


<html>
<head>
<style>
<!--
body{font-family:宋体; font-size:12px; padding:0px; margin:0px;}
.showWindow:hover{color:#000000}
.win_bg{background:#000000; position:absolute; left:0px; top:0px; width:100%; height:100%; z-index:998;}
.winTitle{background:#9DACBF; height:20px; line-height:20px}
.winTitle .title_left{font-weight:bold; color:#FFF; padding-left:5px; float:left}
.winTitle .title_right{float:right}
.winTitle .title_right a{color:#000; text-decoration:none}
.winTitle .title_right a:hover{text-decoration:underline; color:#FF0000}
.winContent{padding:2px;}
-->
</style>
<script src="dsd.js">

function demo(width,height,bgcolor,fontcolor,opacity,Title,content,conttype){
  
  if(document.getElementById("divWin"))
  {
   $("divWin").style.zIndex=999;
   $("divWin").style.display="";
  }
  else
  {
   var objWin=document.createElement("div");
   var objwin=document.getElementById("divWin");
   objWin.id="divWin";
   objWin.style.position="absolute";
   objWin.style.width=width;
   objWin.style.height=height;
   objWin.style.border="2px solid #AEBBCA";
   objWin.style.background=bgcolor;
   objWin.style.color=fontcolor; 
   objWin.style.top= (document.body.clientWidth-objWin.offsetHeight)/3.5+"px"; 
   objWin.style.left=(document.body.scrollLeft + (document.body.offsetWidth-objWin.offsetWidth)/3.5)+"px"; 
   
  // objWin.style.left=(document.documentElement.offsetWidth-objWin.offsetWidth)/2 + document.documentElement.scrollLeft+"px";
  // objWin.style.top=(document.documentElement.clentHeight-objWin.offsetHeight)/2 + document.documentElement.scrollTop+"px";    //objWin.style.top="30%";
   //objWin.style.left="15%";
   objWin.style.zIndex=999;
      
   document.body.appendChild(objWin);
  }
  
  if(document.getElementById("win_bg"))
  {
   $("win_bg").style.zIndex=998;
   $("win_bg").style.display="";
  }
  else
  {
   var obj_bg=document.createElement("div");
   obj_bg.id="win_bg";
   obj_bg.className="win_bg";
   obj_bg.style.filter = "alpha(opacity="+opacity+")";
   obj_bg.style.opacity = opacity/100;
   document.body.appendChild(obj_bg);
  }
  if (conttype=='url'){
var str="";
str+='<div class="winTitle"><span class="title_left">'+ Title + '</span><span class="title_right"><a href="javascript:closeWindow()" title="单击关闭此窗口">关闭</a></span></div>';  //标题栏
str+='<div class="winContent" ><iframe src='+ content +' height=95% width=100%></iframe></div>';  //窗口内容
$("divWin").innerHTML=str;
  }else{
  var str="";
  str+='<div class="winTitle"><span class="title_left">'+ Title + '</span><span class="title_right"><a href="javascript:closeWindow()" title="单击关闭此窗口">关闭</a></span></div>';  //标题栏
  str+='<div class="winContent">'+ content+'</div>';  //窗口内容
  $("divWin").innerHTML=str;} }
function closeWindow(){

  $("divWin").style.display="none";
  $("win_bg").style.display="none";
  $("divWin").parentNode.removeChild($("divWin"));
  $("win_bg").parentNode.removeChild($("win_bg"));
}
function $(o){
  return document.getElementById(o);
} </script> </head>
<body>
<a class="showWindow" href="javascript:demo(150,200,'#fff','#000',20,'打开内容','http://www.baidu.com','string')">点击这里打开窗口显示内容</a><br />    <a class="showWindow" href="javascript:demo(800,300,'#fff','#000',100,'打开网站','http://www.baidu.com','url')">点击这里打开窗口显示新网站</a><br /> </body>
</html>
这里要求让弹出的div在任何时候都要保持居中状态，包括在改变了窗口大小的时候，应该如何操作，急求