请教js拖动div问题

var Layer='';
var iLayerMaxNum=1000;
var a;
var b;
var c;
document.onmouseup=me;
document.onmousemove=ms;
function Move(Object,event)
{
Layer=Object.id;
if(document.all)
{
document.getElementById(Layer).setCapture();
b=event.x-document.getElementById(Layer).style.pixelLeft;
c=event.y-document.getElementById(Layer).style.pixelTop;
}
else if(window.captureEvents)
{
window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
b=event.layerX;
c=event.layerY+100;
};
if(Layer!="Layer"+a)
{
document.getElementById(Layer).style.zIndex=iLayerMaxNum;
iLayerMaxNum=iLayerMaxNum+1;
}
}
function ms(d)
{
if(Layer!='')
{
if(document.all)
{
document.getElementById(Layer).style.left=event.x-b;
document.getElementById(Layer).style.top=event.y-c;
}
else if(window.captureEvents)
{
document.getElementById(Layer).style.left=(d.clientX-b)+"px";
document.getElementById(Layer).style.top=(d.clientY-c)+"px";
}
}
}
function me(d)
{
if(Layer!='')
{
if(document.all)
{
document.getElementById(Layer).releaseCapture();
Layer='';
}
else if(window.captureEvents)
{
window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
Layer='';
}
}
}
function Close(n)
{
var e='Layer'+n;
document.getElementById(e).style.display='none';
}
function Show(n)
{
var e='Layer'+n;
document.getElementById(e).style.zIndex =iLayerMaxNum+1;
document.getElementById("aspk").style.display = "block";
document.getElementById("aspk").style.zIndex = iLayerMaxNum;
var size = getPageSize();
document.getElementById("aspk").style.width = size[0];
document.getElementById("aspk").style.height = size[1];
}
function Hide()
{
document.getElementById("aspk").style.display = "none";
iLayerMaxNum=iLayerMaxNum+2;
}
function getPageSize()
{
var de = document.documentElement;
var w = window.innerWidth || self.innerWidth || (de&&de.clientWidth) || document.body.clientWidth;
var h = window.innerHeight || self.innerHeight || document.body.clientHeight || (de&&de.clientHeight);
arrayPageSize = new Array(w,h);
return arrayPageSize;
}
为什么一拖动，div就会跑到网页左上角啊？