Search problem, pages displayed as results

Hello.
I have a problem with my search form.
I haven’t notice before, but now I see that when I type something like “Contact” as results appear not only posts but the page “Contact”.
How could I prevent that from happening? Here is the code I’m using:

<form id="searchform" action="<?php echo get_option('home') ?>/"  method="get">
            <fieldset class="search">
	                <input type="text" class="box" name="s" />
          	        <button type="submit" class="submit" title=""></button>
            </fieldset>
            </form>

Should I edit it or use a different script? I want the search form to search for posts titles only.