As sbillard said if you are using some third party script to install than all bets are off. You are best just running setup yourself by pointing your browser at the setup.php file (or the base of your zenphoto install) and rebuilding the info that zenphoto needs by completing that script.
As for your question about the username and password in relation to not creating a new db my "best guess" would be that it wants your db user and db password. Of course it is just a guess.
When in doubt, back up your db and give it a run. That way at the very least you can get back to where you were.
The setup script that comes with zenphoto works quite well to get you up and running. The only real thing you need to do in your case is to get rid of the administrator table and then create your new admin account when zenphoto prompts you for it. Oh, and keep your password somewhere safe so you don't have to do this again.