如何让竖向滚动条位置保持不变

怎么让竖向滚动条位置保持不变
 js代码和css样式：
<script type="text/javascript">
        var BigImgUrl;
        function BackFullScreen(BigImgUrl) {
            
            var scrollTop = document.body.scrollTop;
            var FullScreenDiv = document.getElementById("FullScreenDiv");
            FullScreenDiv.style.width = document.body.clientWidth + "px";
            FullScreenDiv.style.height = document.body.clientHeight + "px";
            FullScreenDiv.style.display = "block";
            var ShowBigImgDiv = document.getElementById("ShowBigImgDiv");
            var ShowBigImgDivPosition;
            ShowBigImgDiv.style.display = "block";
            ShowBigImgDiv.innerHTML = "<img src=\"" + BigImgUrl + "\" width=\"800\" height=\"600\"  border=\"0\" />";
            ShowBigImgDivPosition = document.documentElement.scrollTop;
            if (ShowBigImgDivPosition == 0 || ShowBigImgDivPosition == "") {
                ShowBigImgDivPosition = document.body.scrollTop;
            }
            ShowBigImgDiv.style.top = ShowBigImgDivPosition + ((window.screen.height - ShowBigImgDiv.clientHeight) / 2 - 170) + "px";
            ShowBigImgDiv.style.left = (window.screen.width - ShowBigImgDiv.clientWidth) / 2 - 90 + "px";
        }
        function BackFullScreenHidde() {
            document.getElementById("ShowBigImgDiv").style.display = "none";
            document.getElementById("FullScreenDiv").style.display = "none";
        }
    </script>
    <style type="text/css">
        #ShowBigImgDiv
        {
            position: absolute;
            z-index: 10001;
            display: none;
            cursor: pointer;
        }
        #FullScreenDiv
        {
            position: absolute;
            z-index: 10000;
            display: none;
            background-color: #FFFFFF;
            filter: alpha(opacity=50);
            opacity: 0.5;
            cursor: pointer;
        }
    </style>

html：点击图片显示大图后，滚动条被置顶了，请教要使滚动条位置保持不变 应该怎么解决？
 <td class="img_pic"  style="width: 180px; height: 150px;">
                    <div id="ShowBigImgDiv" onclick="BackFullScreenHidde()" title="点击缩小"></div>
                    <div id="FullScreenDiv" onclick="BackFullScreenHidde()" title="点击缩小"></div>
                <a href="#"><img src="$imgpath/shop_business.jpg" width="150" height="116" onclick="BackFullScreen(this.src)"/></a>
                <p>1、图片样板</p>
                </td>
------解决方案--------------------