Some crude coding
<?php
$rohan = $_SERVER['HTTP_REFERER'];;
$rohan1 = 'http://localhost/projects/Sairo/wordpress/?cat=15';
if ( $rohan == $rohan1 ) {
echo $rohan;
include(TEMPLATEPATH . '/celeblist.php');
}
else {
include(TEMPLATEPATH . '/archivelist.php');
}
?>
Am not too good at PHP but i guess you should use a page referer. That informs the search page to default to the category search even if there are no results from the category.
You should look at plugin that helps you search within a category. The second input field should be ideally be a drop down that lets the user select a city/ town/ country etc. Hope this helps