On Apache server's I can create a .htaccess file with the following code.
AddHandler application/x-httpd-php5 .php
Is there something similar that I can do for a windows server or does php5 need to be enabled site wide?
PHP can be disabled on an individual site via the Handler Mappings for a given site. Information on setting up the handler mapping can be found here, but if it is already enabled on a site, you only have to disable it on the Handler Mapping settings.
You may also want to have a look at the PHP Manaager for IIS on Codeplex.