We delegate increasingly sophisticated and complex tasks to computer programs, and we require them to act autonomously on our behalf in distributed environments (eg the web, the grid). Personal Assistants (PA) are computer programs that can be thought as a very natural metaphor for such systems. Semantic Web Services are self-contained, modular applications that can be semantically described and published over the Web by service providers. This makes it possible for a PA to look for a particular type of service in a directory and invoke it on the fly. However, current service description languages such as OWL-S do not help much the PA to determine the correct input arguments in order to invoke the service and so user intervention is required. We are aimed at minimizing the necessary user intervention by adding new service description features and the use of a dialogue-based grounding mechanism that can be exploited by the procedural semantics capabilities of the provider and the reasoning capabilities of the PA. As a validation we use the proposed approach in an e-service example to compute an insurance premium based on the requirements of the user.