A significant number of brain tumor patients who received radiation or chemotherapy have thinking problems as a result of their treatment. The purpose of this study is to find out if treatment with Aricept (donepezil) may improve some aspects of thinking abilities in patients with brain tumors who received radiation or chemotherapy. This research will also study whether persons having particular genes for a blood-borne substance called apolipoprotein E (APOE) are more likely to have thinking problems after radiation or chemotherapy treatment for their brain tumors. The findings of this study will help us find out whether Aricept can improve thinking abilities after cancer treatment, and whether some of the thinking difficulties may be in part related to having certain genes.