The framework I would use is as follows:

• each navigation session in your site (identified by a cookie or passing a parameter from page to page) constitutes a session-object. This is a standard technique and you may find helpful info with a little search. Sessions should have an expiration time.
• each registered user has a profile and some data associated. The user-object is created on registration and stored somewhere. It doesn't expire (or does, but only after a long time).
• when the session starts and you cannot determine if the user is known to the application, you start with a new "anonymous" user-object linked to the session. You record the data in this user-object.
• If the user logs-in, then his stored user-object is retrieved and his data and preferences are merged with those stored in the current anonymous user-object. This way, if the user has selected a new 'preferred language' or has filled a form before logging-in, the info is not lost but kept for future use.

Hope that helps

Antonio

The stupider the astronaut, the easier it is to win the trip to Vega - A. Tucket