show custom posts types in archives

You can use the following code in your category archive template file.

if (is_category()) {
    global $query_string;
    $post_type = get_query_var('post_type');
    if ( !$post_type ) { // if no post type is already defined
        parse_str( $query_string, $args );
        $args['post_type'] = array( 'post', 'you-custom-post-type-here' );
        query_posts( $args );
    }
}

You can also use the above code in any action that gets run just before the main loop.