How to display posts under one catagory

Hi, I am currently using this snippet which I found on the net in a template

<?php
         $paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
         $args = array (
            'posts_per_page' => $posts_per_page,
            'post_type' => 'post',
            'orderby' => 'title',
            'order' => 'ASC',
            'paged' => $paged
         );
         query_posts($args);
         if ( have_posts() ) {
            $in_this_row = 0;
            while ( have_posts() ) {
               the_post();
               $first_letter = strtoupper(substr(apply_filters('the_title',$post->post_title),0,1));
               if ($first_letter != $curr_letter) {
                  if (++$post_count > 1) {
                     end_prev_letter();
                  }
                  start_new_letter($first_letter);
                  $curr_letter = $first_letter;
               }
               if (++$in_this_row > $posts_per_row) {
                  end_prev_row();
                  start_new_row();
                  ++$in_this_row;  // Account for this first post
               } ?>
               <div class="title-cell"><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></div>
            <?php }
            end_prev_letter();
            ?>
            <div class="navigation">
               <div class="alignleft"><?php next_posts_link('&laquo; Higher Letters') ?></div>
               <div class="alignright"><?php previous_posts_link('Lower Letters &raquo;') ?></div>
            </div>
         <?php } else {
            echo "<h2>Sorry, no posts were found!</h2>";
         }
         ?>

      </div><!-- End id='a-z' -->

Can anyone tell me how to edit it to show only posts from 1 category?