How to Exclude A WordPress Category From Search Results

Insert the code to search.php after  <?php get_header(); ?>
<?php
$page = (get_query_var(‘paged’)) ? get_query_var(‘paged’) : 1;
$s = get_query_var(‘s’);
query_posts(“s=$s&cat=-3&paged=$page”);
?>

-3 means category ID -3 is excluded.