My colleague Wes has quite often discussed the benefits of statins for fighting Alzheimer's, particularly since the benefits it seems to offer for the cardiovascularly challenged seem transferable to the case of early-stage amyloid formation in the brain.
At the cellular level, what actually appears to happen is that protein structures proliferate. Rather than being broken down by natural processes, these proteins accumulate like layers of saline sediment in a formerly verdant valley, gradually dampening cognitive activity.
Statins appear to act by enhancing blood flow and viscosity which may prevent the early stages of the the protein 'tangle' accumulation. The Archives of Neurology reports on statins in a pilot study that was just published.