随滚动条浮动的链接块层

<script type="text/javascript">
    function scrollImg() {
        var posX, posY;
        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;
        }

        var ad = document.getElementById("ad");
        ad.style.top = (posY + 100) + "px";
        ad.style.left = (posX + 50) + "px";
        setTimeout("scrollImg()", 100);
    }
</script>
<div id="ad" style="position: absolute; background: #eeeeee; text-align: center">
    <div class="well sidebar-nav">
    <ul class="nav nav-list">
        <li class="nav-header">Hello,Chris!</li>
        <hr />
        <li class="nav-header"><span style="color:red;">Hot Links</span></li>
        <li><a href="#">DART</a></li>
        <li><a href="#">DARTboard</a></li>
        <li><a href="#">Lowes.com</a></li>
        <li><a href="#">MPT</a></li>
    </ul>
        </div>
</div>
<script type="text/javascript">
    scrollImg();
</script>