Are you sure you gave us the correct URL/filename? The link you posted resulted in a 404 error, not a 403 as expected.
Beyond that, I would check what NogDog said - if you have any script on your site that does a sort of dynamic include (e.g. something similar to include($_GET['page'] . '.php')) then that's most likely the culprit; many programmers make the 'newbie' mistake of forgetting to properly sanitize the incoming parameter before throwing it into a filesystem path.