I assume you’ve just upgraded to 1.5.
Your base index.php should be:
<?php
/* Short and sweet */
define('WP_USE_THEMES', true);
require('./wp-blog-header.php');
?>
To disable themes, change it to:
<?php
/* Short and sweet */
define('WP_USE_THEMES', false);
require('./wp-blog-header.php');
?>
Eventually, though, you may want to consider using Themes. Themes and the various Template files they support make formatting specific portions of your site easier.
If you don’t want to use many themes, then just don’t change whatever you have.
