Get Posts by Custom Field

I’m trying to get posts with a custom field. Currently I’m using the PHP below which works but outputs ALL of them. I want to only output the latest post, but when I use the query posts in the first line it makes it disappear.

<?php query_posts($query_string); query_posts('showposts=1'); ?>
<?php if (have_posts()) : while (have_posts()) : the_post();
$customField = get_post_custom_values("lead-story");
if (isset($customField[0])) {
//Custom field is set, display post info

$values = get_post_custom_values("Image"); if ( $values != '' ) {
echo '<a href="'; the_permalink(); echo '" rel="bookmark" title="Permanent Link to '; the_title(); echo '"><img src="'; echo get_option('home'); echo '/wp-content/uploads/'; echo $values[0]; echo '" alt="" id="leadpic" class="feature-img" /></a>'; } 

echo '<a href="'; the_permalink(); echo '" rel="bookmark" title="Permanent Link to '; the_title(); echo '" class="lead-story-title">'; the_title(); echo '</a><br /><span class="smalltext">'; the_time('F jS, Y'); echo '</span><p>'; echo substr(get_the_excerpt(), 0, 150); echo '...<br /><a href="'; the_permalink(); echo '" rel="bookmark" title="Permanent Link to '; the_title(); echo '">Read More &raquo;</a></p>';
}
endwhile;
endif;
?>