Custom default installation

I need to customize the default WP installation. For example, I would like to create a few categories, posts, and pages created upon installation. I found this post on the WordPress MU website, “Change default category and post“, but I couldn’t find the appropriate file for the regular WP install. I found a few lines of code in the /includes/upgrade.php file that appear to be where I want to start (look towards line 60).

Does anyone have any suggestions as to where I might find some documentation on this? Is the /includes/upgrade.php file a good start?

Best wishes!