Adding a search form on every category page?

I want to add a searchform in all my categories to search only the post on the current category.

<form method="get" id="searchform" action="<?php bloginfo('home'); ?>/">
<div><input type="text" value="<?php echo wp_specialchars($s, 1); ?>" name="s" id="s" />
<input type="hidden" name="cat" value="22" />
<input type="submit" id="searchsubmit" value="Search" />
</div>
</form>

How to modify this value to be for the current category page without need to make a template for every category?
<input type="hidden" name="cat" value="22" />

THNX ??