When I register as a new user I get an error.
An illegal choice has been detected. Please contact the site administrator.
If I reenter my password fields, and press create new account then it lets me register. Ideas?
An illegal choice typically means that the Form API received something it didn't expect when the form was processed. As Stan Ascher says, make sure you don't have modules enabled which modify the login process. You can tell if there are functions which end in _user or _form_alter in those modules.