Exclude a category from random post

Hi

I would like to exclude a category from random post in the home page

Now I’m using this code:

<?php $posts = get_posts(‘orderby=rand&numberposts=25’); foreach($posts as $post) { ?>
<?php } ?>

How can I exclude a category?

This is the site:coltellidacucina.biz

Thx for help.
D.