Instead of using
<?php query_posts('cat=1') ?>
Use
<?php if (have_posts()) : ?>
<?php if (is_month()) {query_posts('year='.get_the_time('Y').'&monthnum='.get_the_time('m').'&cat=1&showposts='.get_option('posts_per_page')); ?>
<?php } ?>
<?php while (have_posts()) : the_post(); ?>
<?php endwhile; ?>
<?php endif; ?>
