May be you just wondering how to filter the WordPress search result by category. It's possible by adding some function to your wordpress, but if you don't familiar with PHP, I just write a plugin for you called WP Search Filter.
This plugin is simply add a dropdown categories next to the search form. If one of the category is selected, then it will only search on that category.
Actually it's also possible to filter the search result by custom field , but I will do it later 
Installation
Just install the plugin and activate. Make sure your search form is called from:
<?php
get_search_form();
<p>?> Demo:
Please see the live demo at the top right of this website.
Download