This study will evaluate the safety and effectiveness of donepezil (Aricept) for treating mild dementia in patients with Parkinson's disease. Donepezil is approved for treating patients with Alzheimer's disease, whose memory and cognition problems are similar to those of patients with Parkinson's disease who are affected by dementia. Donepezil prevents the breakdown of a chemical messenger called acetylcholine, which is involved in memory and other cognitive functions, and may improve cognition in patients.