Problem with next previous buttons

This is the code from index.php. It works great except on the front page. As you can see in the first line, i have specified that category 250 should not display on the front page. The query works, but the next previous buttons don’t work. When i click, is shows the same 10 posts over and over again.

<?php if (is_front_page()) {	  query_posts('cat=-250');  } ?>
<?php while ( have_posts() ) : the_post(); ?>
<article class="entry home">
<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
<div class="fixedheightentry"><?php excerpt('50'); ?></div>
<span class="post-date"> Posted by <?php the_author_link(); ?> on <?php the_time('F jS, Y') ?></span>
</div>
<span class="meta">
<a class="read-more-s" href="<?php the_permalink()?>" title="Permanent Link to <?php the_title_attribute(); ?>">Read More</a>
</span>
</article>
<?php endwhile; ?>

<span class="new_posts"><?php previous_posts_link('Newer',''); ?></span>
<span class="old_posts"><?php next_posts_link('Older',''); ?> </span>