Search Results by Category

On the search results, page how is it possible to just define results by certain category? So when user searches he is limited by default to searching set categories..

I tried the following it didn't work, looking for some guidance

<div class='codetop'>CODE</div><div class='codemain' style='height:200px;white-space:pre;overflow:auto'><?php if ($_REQUEST['do'] == "search" or $_REQUEST['dosearch'] == "yes")
            { $subaction = "search"; $dosearch = "yes"; include("cms/search.php"); } else { $category = "1"; include("cms/show_news.php"); } ?></div>