you’ve got it slightly wrong – you have to include a minus symbol (-) before the cat ID to tell it to exclude it.
here is my form (without the styling):
<form method="get" id="searchform" action="<?php bloginfo('home'); ?>/">
<input type="hidden" name="cat" value="-1" />
<input type="text" value="<?php echo wp_specialchars($s, 1); ?>" name="s" id="s">
<input class="searchbutton" type="submit" id="searchsubmit" value="SEARCH">
</td></tr></table></form>
