首先css解决一下ie6不支持position:fixed;的缺陷。
css
===

#menu {
            position: fixed;
            top: 100px;
            left: 50%;
            margin-left: 400px;
            width: 50px;
        }
 * html, * html body {
        background-image: url(about:blank);
        background-attachment: fixed;
        }

* html #menu {
        position: absolute;
        bottom: auto;
        top: expression(100+((e=document.documentElement.scrollTop)?e:document.body.scrollTop)+'px');
        }

js:

 $(document).ready(function () {
            $(window).scroll(function () {
                var items = $("#content").find(".item");
                var menu = $("#menu");
                var top = $(document).scrollTop();
                var currentId = ""; //滚动条现在所在位置的item id
                items.each(function () {
                    var my = $(this);
                    //m.offset().top代表每一个item的顶部位置
                    if (top > my.offset().top) {
                        currentId = "#" + my.attr("id");
                    } else {
                        return false;
                    }
                });

                var currentLink = menu.find(".current");
                if (currentId && currentLink.attr("href") != currentId) {
                    currentLink.removeClass("current");
                    menu.find("[href=" + currentId + "]").addClass("current");
                }
            });
        });