Stop IPsec Policy Agent
My Win Vista laptop's disk activity light kept flashing to no end. Something was killing my disk. I opened "Reliability and Performance Monitor" and found a service called "svchost.exe (LocalSystemNetworkRestricted)" was to blame. After some poking around I found this's not Superfetch, Windows Search, or font Cache. It's "IPsec Policy Agent". I stopped this service, my "Disk % Highest Active Time" dropped from 100% to 0-1%, and stayed here. The disk activity indicator light stopped flashing. I disabled this service. If later I need to do VPN, I will probably have to re-enabled it again. Hope I remember it.