The size and complexity of modern instructional systems, which are based on the World Wide Web, bring about great intricacy in their crafting, as there is not enough knowledge or experience in this field. This imposes the use of new instructional design models in order to achieve risk-mitigation, cost and time efficiency, high pedagogical quality of the end product, which will capitalise on the potential of the networked technologies. This paper presents a model for constructing such systems, using a problem-solving approach that defines the solution of an instructional problem in both a pedagogical and a technical manner in a holistic view. This model tries to standardise the development of such systems supporting reusability at the design as well as the implementation level. The pedagogical or non-technical solution is comprised of five interconnected sets of learning elements: the learning objectives, the didactic events, the syllabus, the assessment procedure and other issues like prerequisites, fees, technical constraints, etc. This first form of the solution is then transformed into a technical form that contains implementation sub-systems: the human subsystem, the learning resources subsystem, and the technical infrastructure subsystem. These subsystems are perceived as independent, autonomous entities that interact with each other through critical interfaces in a holistic fashion. The technical solution is therefore the system architecture of the web-based instructional system, which is engineered to satisfy the requirements specified by the pedagogical solution. The proposed model has been empirically derived from actual web-based instructional systems, supporting undergraduate and postgraduate courses in the Computer Science discipline. One of these courses is presented in this paper, serving as a case study, and exhibiting the formulation of both types of the solution. Furthermore, in order to evaluate the effectiveness of the Web-based instructional systems, constructed with the proposed model, a summative evaluation method is employed that makes use of pre-test and post-test questionnaires. This evaluation method is briefly presented, together with a summary of its results on the case study course. Finally the proposed model is examined in the context of current standardization efforts in the field of Learning Technology Systems. Specifically it is demonstrated how the model can conform to the Learning Technology System Architecture draft standard of IEEE Learning Technology Standards Committee. In conclusion, the model has shown encouraging results in the four systems it has been applied to. We mean to further elaborate on the model by employing it, to construct more instructional systems. We also aim in coding design experience in the field, in the well-established form of design patterns.