[WebMethod]
    public bool ChangePassword(string userName,string password,string validCode)
    {
        string msg = "";
        if (validCode != getOriginValidCode())
        {
            msg = "无效的BI确认码!";
            throw new Exception(msg);
           ;
        }
        string adPwd = System.Configuration.ConfigurationManager.AppSettings["adPwd"];
        string adUser = System.Configuration.ConfigurationManager.AppSettings["adUser"];
        DirectoryEntry myDirectoryEntry;
        myDirectoryEntry = new DirectoryEntry("WinNT://" + Environment.MachineName, adUser, adPwd);
        DirectoryEntry u = myDirectoryEntry.Children.Find(userName);
        if (null == u)
        {
            msg = "无效的用户";
            throw new Exception(msg);          
        }
        u.Invoke("setPassword", password);
        u.CommitChanges();
        if (msg != "")
        {
            throw new Exception(msg);
        }
        return true;
    }

<authentication mode="Windows"/>
<identity impersonate="true" userName="userName" password="password"/>