 The purpose of the research is to see how simvastatin affects a substance in the body called beta-amyloid. Beta-amyloid is found in the brain and in the liquid around the brain and spinal cord. High amounts of beta-amyloid may be associated with a greater risk of getting Alzheimers disease. This study will see if simvastatin can lower the amount of beta-amyloid in the spinal fluid. This study will also see if simvastatin affects memory and thinking, blood flow in the brain, and blood vessel function. The investigators hope that future studies show whether simvastatin might prevent memory loss and decrease the chance of developing Alzheimers disease. 