Need help adding checkbox/radio button values to an input for xml string!!!

Need help adding checkbox/radio button values to an input for xml string!!!

Hi, I can't seem to get this working correctly. I have several radio buttons and checkboxes. Based on them being checked I need to add that value to an input (which will be hidden) to be passed as a form parameter. The input they are added to is wrapped in xml tags. Currently it is simply overwriting each value, not adding them to the string. One value "IKNTK" needs to be passed regardless.

<script type="text/javascript">
    $(function() {
        $(':radio,:checkbox').click(SetServices);
    });

    function SetServices() {
        var services = '';
        if($("#Standard").is(':checked')) {
            services += '<SERVICE><SERVICECD>' + '1KNTK' + '</SERVICECD></SERVICE>';
        }
        if ($(this).is(':checked')) {
            services += '<SERVICE><SERVICECD>' + this.id + '</SERVICECD></SERVICE>';
        }
       
           
        $('#SERVICES').val('<SERVICES>' + '<SERVICE><SERVICECD>1KNTK</SERVICECD></SERVICE>' + services + '</SERVICES>');
    }
</script>

Please help me figure this out!
Kane Leins