I have an old WordPress.org blog (as well as a WordPress.com blog) and now have installed a new WP blog. I thought perhaps the login id and password would carry over, but that is SO obviously not the case. It's clear enough when I looked at SpencerP's reference a few hours ago to modifying one's MySQL database. Well, I've done that, and as yet do not have any response when I attempt to login. I'm trying to login in as http://www.mynewdomain.com/wp-admin . The newdomain has WP installed in the domain directory (not an add-on file, as on the old one).
There is probably an instruction that I failed to see, this time around, that would have me setting the login and password.
On the other hand, I learned a little more about MySQL by monkeying with the database for WP.