We've wrote a website so that users can purchase ticket (for concerts). For users convenience we wrote user temporary registration and auto login systems. So users don't need to Sign in/Sign up to purchase ticket.
For more features(e.g tracking his/her purchase) sign up/sign in is needed, but I don't know where is the best place to put Sign in/Sign up box on the website. I think if I put such a box in header section(e.g) newbie user will think sign up is mandatory.
You can put a signup form in the summary screen after the actual purchase. The text could be something like "Great, you have bought your tickets and we sent a copy to your email address. If you wish to track them, change your booking later, or , you can set up a ticket book below". Then show a simple sign up form with textbox for email address (prefilled since you presumably have the email from the purchase), text box for password, and "create my ticket book" button.
The advantage is that people are not forced to sign up, its pretty clear what the benefits/tradeoffs are, and they will be more receptive to signing up after they have already done most of the work. Of course, the whole login system should use email addresses as identificators, i.e. don't force the user to make up yet another username.
Something that I would consider is automatically making them an account as they visit (but instead of a password it's by cookie), which I'm assuming is similar to what you have already.
Then somewhere where it's not sticking out there too much, but still noticeable, have a message saying that they can change their password and display name (which would then actually "create" the account).
You'd want to stick it in a sidebar or something. Where it's not thought to be mandatory, but where it's still noticed.
