You can do it in at least 2 ways: give the user a cookie when they log in, and only allow those that have a cookie to procedure to other pages, or use hidden fields that store some sort of session ID for the user. Both topics have been covered often here at PM, and you should be able to find more help using Super Search, as well as by reading merlyn's columns on web design.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain