next/previous posts link dont work in the loop

I run the loop like this to exclude one category and show 5 posts in the frontpage.

<?php query_posts('cat=-1&showposts=5'); ?>
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
...
<?php the_excerpt() ?>
...
<?php endwhile; ?>

<div class="navigation">
<div class="alignleft"><?php next_posts_link('« Previous Entries') ?></div>
<div class="alignright"><?php previous_posts_link('Next Entries »') ?></div>
</div>

<?php else : ?>

<h2>Not Found</h2>
Sorry, but you are looking for something that isn't here.

<?php endif; ?>

But the next_posts_link and previous_posts_link cannot correctly.I got 8 sample posts (1 post has been excluded),however,just one page shows up.

Can anyone gimme a hand,if you want more details,I’ll post as more as possible.