Searchform for searching specific categories

A simple search in codex shows there are some parameters you can set to exclude some categories for example :

 <?php $args = array(
'show_option_all'    => '',
'show_option_none'   => '',
'orderby'            => 'ID', 
'order'              => 'ASC',
'show_count'         => 0,
'hide_empty'         => 1, 
'child_of'           => 0,
'exclude'            => '',
'echo'               => 1,
'selected'           => 0,
'hierarchical'       => 0, 
'name'               => 'cat',
'id'                 => '',
'class'              => 'postform',
'depth'              => 0,
'tab_index'          => 0,
'taxonomy'           => 'category',
'hide_if_empty'      => false
); ?>