<form id="Form1" runat="server" action="Index.aspx" method="post">
    <div id="loginForm">
        <table>
            <tr>
                <td>
                </td>
                <td>
                    <input type="hidden" value="Login" name="Login" />
                </td>
            </tr>
            <tr>
                <td>
                     用户名：
                </td>
                <td>
                    <input name="userName" />
                </td>
            </tr>
            <tr>
                <td>
                     密码：
                </td>
                <td>
                    <input id="password" name="password" />
                </td>
            </tr>
            <tr>
                <td>
                    <input type="submit" value="登陆" />
                </td>
                <td>
                    <input type="reset" value="取消" onclick="Reset()" />
                </td>
            </tr>
        </table>
    </div>
    <div id="block">
    </div>
    </form>form最后的id=block的div用来屏蔽后面的内容

/**//*登陆Form*/
#loginForm
{
     border:solid 1px #AED5FD;
     position:absolute;
     top:35%;
     left:40%;
     z-index:2000;
     background-color:#EAF4FE;
}
/**//*屏蔽Div*/
#block
{
     background-color:#CCCCCC;
      position:absolute;
      top:0px;
      left:0px;
     z-index:1000;
     display:block;
     width:100%;
     height:210%;
    /**//*设置屏蔽div的透明度*/
     filter : progid:DXImageTransform.Microsoft.Alpha ( enabled=true , style=0 , opacity=40);
     }

function ShowLogin()
{
var ele=document.getElementById("loginForm");
var block=document.getElementById("block");
ele.style.display="";
block.style.display="";
}
function Reset()
{
     document.getElementById("loginForm").style.display="none";
     document.getElementById("block").style.display="none";

}
window.onload=Reset();