Change default wordpress query to orderby title

in your archive.php, find the code:

if (have_posts()) : while (have_posts()) : the_post();

and replace it with:

$cat_posts = new WP_Query($query_string."&orderby=title&order=ASC");
if ($cat_posts->have_posts()):
    while($cat_posts->have_posts()):
        $cat_posts->the_post();