Alternative suggestion. Allow WP to install using the generic admin user. Then install, use and then delete the Change Admin Username plugin. It’s much the easiest option.
Easiest steps:
1. Install blog with default logins (admin and random password).
2. Immediately login and create a new user with your preferred details (ignore the message saying that you should change the auto-generated password). Set this new user’s role to Admin.
3. Logout of Admin and login with the new user. Delete Admin.
Optionally, you will be asked to delete or transfer existing data from Admin to another user. I choose to delete them, because the default data is just some links to WordPress, a generic About page, a simple Welcome post and an example comment. Those are to be deleted anyway, you may just choose to delete them during this process.
Another alternative solution, if you are comfortable working directly with the database: http://wpfirstaid.com/2010/02/your-username-can-be-changed/
NB: Remember to make back-ups! Always!
