网页右侧的漂浮广告代码，随滚动条滚动

HTML代码(即需要漂浮的广告)：
<div class="christmas_ad" style="width:155px; height:220px; position:absolute; top:250px; right:25px;">
<a href="javascript:void(0)" ><img src="/themes/mall/wowsai/topics/images/christmas_2012/christmasAcvance.png" border="0" /></a>
</div>
js代码：前提是必须要引入jquery文件
<script type="text/javascript">
$(function(){

$(window).scroll(function(){

    var scrollTop = document.body.scrollTop || document.documentElement.scrollTop || 0;

    $(".christmas_ad").stop();

    var scrollTop2 = (scrollTop+160) - $(".christmas_ad").position().top;

    if(scrollTop> 440){

        $(".christmas_ad:not(:animated)").animate({top:"+="+scrollTop2+"px"},1000);

    }else{

        $(".christmas_ad").css("top",370+"px");

    }

})


}) 
</script></pre>