You need to enable guest access with no password. I have noticed however, that if someone has an account and has logged in before, browsers will cache that information and still present a log in page. But someone that has never logged in, they should be directed straight to that page.
I am not exactly sure what you mean, but maybe Emma is right on target.
As you think about your needs, I wanted to share with you that I make course information public on my Moodle's frontpage (hit the "i" icon to the right of a course.) If a student has received my "Welcome" email with course code, they can click on a course and enroll themselves. This is simply Moodle's self-enrollment feature. I am offering this as another idea for you to consider.