Problems with Index

I have a website with WordPress 2.5.1 running at http://www.caa-cvd.cl

The problem is that I want the index.php to show only one category instead of every post that’s created, so I entered this code into index.php:

(...)
	<?php if (have_posts()) : ?>

	<?php query_posts('category_name=general&showposts='.get_option('posts_per_page')); ?>

		<?php while (have_posts()) : the_post(); ?>
(...)

The problem appears when I reduce the ammount of post per page, so the “Previous posts” link appears, and when I try to click it, the posts don’t change, it appears as the page haven’t changed. Also, the URI when I click the button is
http://www.caa-cvd.cl/index.php/page/2/
so it should be working.
I think it’s a problem with the loop, but I don’t know how to fix it. Any ideas?

Thanks in advance,