The following instructions are applicable to to 1.5 Joomla. For Joomla 1.6 and higher just use the ItemID option in the Search Module.

By default Joomla! Search results are shown on the Home Page template. It is possible to change this by adding a hidden field to the search module.

Within your homepage template folder create a new directory called “html” and then “mod_search”.

Copy the “default.php” file from /modules/mod_search/tmpl/ into your newly created “mod_search” folder under your template folder.

Open this file at near the bottom just before the end of the form add:

<input type="hidden" name="Itemid" value="5" /> 

Save and upload to your site. You should now see your site-wide template loading. Change the value of the Itemid as needed.