$query_string Wordpress issue

<?php if(is_tag()) { ?>
<?php
global $query_string;
query_posts( $query_string . '&cat=27' );
while(have_posts()) : the_post();
    echo '<div class="indent"><div class="title"><a href="';
    echo get_post_meta($post->ID, 'link', true);
    echo '" target="_blank" title="Link to ';
    the_title();
    echo '"><img src="';
    echo get_post_meta($post->ID, 'thumbnail', true);
    echo '" class="thumbnail" alt="Thumbnail of ';
    the_title();
    echo '" width="120" height="160" /></a><h2><a href="';
    echo get_post_meta($post->ID, 'link', true);
    echo '" rel="bookmark" title="Link to ';
    echo the_title_attribute();
    echo '">';
    the_title();
    echo '</a></h2></div></div>';
endwhile; ?>
<?php } ?>