Windows 7: ReadyBoost (sysmain) has been turned off

Today I encountered an event that when I go to Properties of a flash drive to configure ReadyBoost, it says ReadyBoost is not enabled because the service responsible for ReadyBoost (sysmain) has been turned off on this computer. An administrator can turn the sysmain back on.

To fix it, Superfetch service needs to be started and configured to automatically start.