DIV随窗体的滚动,始终居于屏幕中心

 <script type="text/javascript" language="javascript">
    function modelessAlert(Msg,Date,Content,msgID)
    { 
        document.getElementById("ctl00_Content1_BackPost").style.display="block";
        document.getElementById("ctl00_Content1_lblResult").innerHTML="您正在回复["+Msg+"]于["+Date+"]发表的留言["+Content+"]";
        document.getElementById("ctl00$Content1$txtBackUser").innerHTML="";       
        document.cookie="msgID="+escape(msgID);
        sc1("ctl00_Content1_SearchResult")      
    } 
    function message(User_ID,msg)
    {   
        document.getElementById("ctl00_Content1_divMessage").style.display="block";
        document.getElementById("ctl00_Content1_Label1").innerHTML=msg;
        document.getElementById("ctl00$Content1$txtMessage").innerHTML="";
        document.cookie="User_ID="+escape(User_ID); 
        sc1("ctl00_Content1_divMessage");
    }
    </script>
    <SCRIPT LANGUAGE="JavaScript">
function sc1(name){
 document.getElementById(name).style.top=(document.documentElement.scrollTop+(document.documentElement.clientHeight-document.getElementById(name).offsetHeight)/2)+"px";
 document.getElementById(name).style.left=(document.documentElement.scrollLeft+(document.documentElement.clientWidth-document.getElementById(name).offsetWidth)/2)+"px";
// alert(document.getElementById(name).style.top);
}
</SCRIPT>
<SCRIPT LANGUAGE="JavaScript">
<!--
function scall(){
 sc1("ctl00_Content1_SearchResult");
 sc1("ctl00_Content1_divMessage");
}
window.onscroll=scall;
window.onresize=scall;
window.onload=scall;
//-->
</SCRIPT>