OK, so let’s get everything straight right away; we’re talking about a completely fresh installation; no plugins and no themes are installed, nothing at all. There’s nothing to turn off, and to see if it’s working then. If you’re getting this kind of behavior, then this post will probably help you.
Every time I encountered this problem, it was always due to WordPress being installed in a sub-folder on a hosting account (meaning, addon domain), where an existing WordPress installation was already active in the root folder. Then, the .htaccess file from the existing WordPress installation messes things up. How to fix it? Very easily; change the permalinks of your new (sub-domain) installation and you’re set!