When many users consult service desks simultaneously, these typically saturate. This causes the customer attention to be delayed more than usual. To increase the amount of human agents is a costly process for organizations. All this has motivated the design of a knowledge-based system that automatically assists both customers and human agents at the service desk. Web technology was used to enable clients to communicate with a software agent via chat. Techniques of Natural Language Processing were used for the software agent to understand the customer requests. The domain knowledge used by the software agent to understand customer requests has been codified in an ontology. A rule-based expert system (ES) was designed to perform the diagnostic task. This paper presents a knowledge-based system allowing client to communicate with the service desk through a chat system using free text. Evaluations conducted with users have shown an improvement in the attention of service desks when the software developed is used.