The primary aim of this study is to determine the safety and efficacy of quetiapine (Seroquel) for the treatment of psychosis and/or agitation in patients with primary dementia complicated by coexistent parkinsonism, or patients with Parkinson's disease with dementia [PDD] who have episodes of agitation or psychosis. The secondary aim is to determine the safety and tolerability, particularly the influence on parkinsonism, of quetiapine when used to treat psychosis and/or agitation in patients with dementia complicated by coexistent parkinsonism.