var mybg = document.createElement("div");
    //弹出层
    function d_x(){  
         
    var h = document.body.clientHeight; 
    var w = document.body.clientWidth;
       
    myAlert = document.getElementById("alert"); 
    myAlert.style.display = "block";  
    myAlert.style.position = "absolute";
     
    
    myAlert.style.top = (h/2)+"px";  
    myAlert.style.left = (w/2)+"px";  
    //myAlert.style.marginTop = "-200px";  
    //myAlert.style.marginLeft = "-250px";  
    
    mybg.setAttribute("id","mybg");  
    mybg.style.background = "#000";
    mybg.style.display = "block"; 

    mybg.style.width = "100%"; 
    mybg.style.height = h+"px";  
    mybg.style.position = "absolute";  
    mybg.style.top = "0";  
    mybg.style.left = "0";  
    mybg.style.zIndex = "500";  
    mybg.style.opacity = "0.5";  
    mybg.style.filter = "Alpha(opacity=40)";  
    document.body.appendChild(mybg);  
    document.body.style.overflow = "hidden";   

    //myAlert.style.position = "absolute";
    myAlert.style.position = "fixed";  //不过对IE6-不起作用，你需要使用计时器移动层

------解决方案--------------------
$(window).bind("scroll",function(){改层的位置})

------解决方案--------------------
ie6不支持position:fixed.写特例吧
可以看看这个http://www.cnblogs.com/hooray/archive/2011/05/20/2052269.html