string file = @"c:\bb.xml";  
        // declearation 
       // parent node

        XmlElement subnode = xdoc.CreateElement("subnode");
       // Checking File Exist or not
        if (File.Exists(file))
        {
            xdoc.Load(@"C:\bb.xml");
            xdoc.DocumentElement.AppendChild(userinfo(txtuser.Text, txtpass.Text));
            xdoc.Save(file);
        }
        else
        {
            subnode.AppendChild(userinfo(txtuser.Text, txtpass.Text));
            xdoc.AppendChild(subnode);
            xdoc.InsertBefore(xdoc.CreateXmlDeclaration("1.0", "utf-8", null), subnode);
            xdoc.Save(file);
        }


// then create the xml node using this methods


XmlNode node = xdoc.CreateElement("Info");
        XmlNode user_node = xdoc.CreateElement("user");
        user_node.InnerText = user.ToString();
        node.AppendChild(user_node);
        XmlNode pass_node = xdoc.CreateElement("password");
        pass_node.InnerText = pass;
        node.AppendChild(pass_node);
        return node;