It is already established in previous research that preventive maintenance can slow the worsening process of a repairable system and restore the system in a younger state. Kumar et al.11 studied the effect of preventive maintenance on computer systems with independent hardware and software failure. Kumar and Malik10 carried out the cost-benefit analysis of computer systems with conducting preventive maintenance after maximum operation time. Kumar and Malik13 developed a reliability model for a computer system by using the concept of priority to hardware repair over replacement of the hardware components. Recently, Malik and Munday14 suggested a stochastic model for computer systems with the provision of redundant hardware component. But so far no work related to reliability modeling of Computer system has been reported in the literature of reliability using the concept of imperfect fault detection of hardware components. In view of the above practical utility of computer systems, an effort has been made in the present work to obtain several performance measures of a computer system with independent hardware and software failures subject to imperfect fault detection of hardware components. For this purpose, a stochastic model is developed by using regenerative point technique. The following measures of system effectiveness are obtained: ¾