MINWii, a new serious video game targeting Alzheimer and demented patients, is a simple music therapy tool usable by untrained care givers. Its goal is to improve patients' self-image (renarcissization) to reduce behavioral symptoms, which are an important cause of institutionalization. With MINWii, elderly gamers use Wiimotes to improvise or play predefined songs on a virtual keyboard. We detail our design process, which addresses the specific features of dementia: this iterative refinement scheme, built upon qualitative, small scale experiments in a therapeutic environment, led to a shift of MINWii's original focus from creativity to reminiscence. A large majority of our patients, with mild to moderate dementia, expressed a strong interest in our system, which was confirmed by feedback from the care givers. A controlled therapeutic study of MINWii is currently under way, which investigates its impact on behavior and quality of life in a hospital setting.