index files in subdirectories

I have a server set up with Apache/2.0.55 (Win32),  PHP/5.1.2, and Perl (ActivePerl 5.8.8 Build 817).

My main index page (index.php) resides in my DocumentRoot directory and is served by entering the url without the need to add index.php to the end.

This page has links to my company's bugzilla page and wiki page.

When setting up the links, if I add index.cgi (for Bugzilla) and index.php (for wiki) to the end of the urls the browser navigates to those pages and it all works ok but it I put a link in like http://blah.com/blah/ I get a forbidden message.
Apache wont serve the default pages for subdirectories of my DocumentRoot folder.

Does anyone know why this might be and how I may resolve this?

Karl