Traditional decision support systems give not enough possibilities of intervention to the user. In complex situations, decision is not structured and it becomes primordial to design intelligent and cooperative systems allowing a joint resolution of problem and dynamic sharing of the tasks between the user and the system according to problems to be solved and an appropriate cooperation mode. We propose, in this paper, a cooperative architecture for intelligent decision support system. The framework embeds expert knowledge within the DSS to provide intelligent decision support, and implements the intelligent DSS using collaboration technologies by putting the decision maker effectively in the loop of such DSS. To this end, besides the conceptual model of the domain, we used a structure based on task-method paradigm for task conceptual modelling. Applicability and relevance of this system are illustrated through a case study where the system and the user cooperate in decision problem solving which consists of identifying boiler defects, diagnosing and suggesting actions of cure