Task modeling has become one of the cornerstones of model-based user interface design. Although different task modeling approaches to user interfaces design have been pushed, ConcurTaskTrees notation is becoming a de facto standard in the design of user interfaces including task-based modeling techniques. In this paper, a task-based approach to user interfaces design is introduced inspired by ConcurTaskTrees (Paternò, 1999). This approach is supported by a tool, namely IDEALXML, that allows for the animation of the specified user interfaces to generate a hi-fi prototype of the future user interface while still in the first development stages.