Patients receive either donepezil or placebo for the first 6 months of the study. In the second 6 months of the study, patients are randomized to receive either donepezil plus GBE or donepezil alone. Positron Emission Tomography (PET), which provides a color-coded image of the brain's blood flow in a particular area, is used to assess brain activity during memory tasks. A PET scan is performed at study entry, 6 months, and 1 year. A vasodilator (a drug that dilates the blood vessels) is administered during the PET imaging procedure to determine whether vascular disease affects memory function. A magnetic resonance imaging (MRI) scan is also taken to view the brain.