如何使层在页面上动态居中

function MiddleCenterDiv()
    { 
        var div=document.getElementById("divBet"); 
        var posX,posY;         
        var top=(window.screen.height -420)/2;
        var left=(window.screen.width-400)/2;
        if (window.innerHeight) {  
            posX = window.pageXOffset;  
            posY = window.pageYOffset;  
        }  
        else if (document.documentElement && document.documentElement.scrollTop) {  
            posX = document.documentElement.scrollLeft;  
            posY = document.documentElement.scrollTop;  
        }  
        else if (document.body) {  
            posX = document.body.scrollLeft;  
            posY = document.body.scrollTop;  
        }  

        div.style.top=(posY+top)+"px";  
        div.style.left=(left)+"px";  
        setTimeout("MiddleCenterDiv()",500);  
    }