A computerized method and system for administering an unannuitized annuity plan having a guaranteed minimum withdrawal payment feature associated with a systematic withdrawal program includes the steps of and system for storing data, determining an amount of a maximum guaranteed withdrawal payment for a prescribed period (e.g., one year), periodically determining the account value, monitoring for an unscheduled withdrawal made under the plan and adjusting the amount of the maximum guaranteed withdrawal payment in response to the unscheduled withdrawal. Guaranteed withdrawal payments are periodically made to the account owner so long as withdrawals do not exceed the maximum guaranteed amount for the period, or the account value is greater than the payment.