怎么样拖动一个div？

Question

  <div id="div3" style="position:absolute;left:10;top:10;z-index:3;background:#a0fbda" onmouseout="drag=0" onmouseover="dragObj=div3; drag=1;">333</div> <div id="div2"  style="position:absolute;left:100;top:100;z-index:2;background:#facd00" onmouseout="drag=0" onmouseover="dragObj=div2; drag=1;">
<div id="div1" style="left:130;top:150;z-index:2;background:#6666FF">111</div>
222
</div><SCRIPT LANGUAGE="JavaScript">
<!--
drag = 0
move = 0function init() {
    window.document.onmousemove = mouseMove
    window.document.onmousedown = mouseDown
    window.document.onmouseup = mouseUp
    window.document.ondragstart = mouseStop
}function mouseDown() {
    if (drag) {
        clickleft = window.event.x - parseInt(dragObj.style.left)
        clicktop = window.event.y - parseInt(dragObj.style.top)
        dragObj.style.zIndex += 1
        move = 1
    }
}function mouseStop() {
    window.event.returnValue = false
}function mouseMove() {
    if (move) {
        dragObj.style.left = window.event.x - clickleft
        dragObj.style.top = window.event.y - clicktop
    }
}function mouseUp() {
    move = 0
}init();
//-->
</SCRIPT>