Nowadays, many companies rely on computer systems and networks for functions such as order entry, customer support, supply chain management, and employee administration. Therefore, reliability and availability of such systems and networks are becoming critical factors for the presentday enterprise or institution. Nevertheless, the task of determining reliability/availability levels for time-dependent systems and networks at the design stage can be: (a) extremely difficult to perform -mainly due to the system/network complexity, and (b) expensive, both in terms of necessary time and money. In this paper, we present a Java-based software, J-SAEDES, which makes use of discrete-event simulation to: (i) estimate reliability and availability of time-dependent computer systems and networks, (ii) identify those components that play a critical role in the system/network reliability or availability, and (iii) obtain additional information on some system/network performance variables. An application example shows some potential uses of this software.