1. A computer system for processing data relating to an investment arrangement having an account value dependent on investment performance and available for withdrawal by an account owner, comprising: a data storage device storing data indicative of a prior period account value, a present account value, a withdrawal factor value, a prior period payment base value, and a guarantee of payments available for a term in a plurality of periods without reduction of the payment base value; a processor in communication with the data storage device; a non-transitory computer-readable medium in communication with the processor, the non-transitory computer-readable medium having computer-executable instructions stored thereon, which instructions, when executed by the processor, cause the processor to: determine a present payment base value based on the present account value and the prior period payment base value, the processor being configured to limit a periodic increase in the payment base value to a maximum percentage; for each of a plurality of periods, including periods after a first withdrawal, determine an available maximum withdrawal without reduction of the payment base value for a current one of the periods based on a greater of: (i) a value based on the withdrawal factor value and the present account value; and (ii) a value based on the withdrawal factor value and the present payment base value; and provide data indicative of the determined available maximum withdrawal amount via a communication bus to a display device for display to a computer system user; and a display device in communication with the communication bus and the processor and configured to display the determined available maximum withdrawal amount.