$(document).ready(function(){
    function enable_disable_form()
    {
        box = $('#form_accepted_terms');
        submit = $('#form_submit');
            
        if ($('#form_accepted_terms').is(':visible') && $(box).is(':checked'))
        {
            $(submit).removeAttr('disabled');
        }
        else
        {
            $(submit).attr('disabled', 'disabled');
        }
    }
    $('#form_accepted_terms').click(enable_disable_form);
});