phantom harddrive activity

I’m getting a lot of phantom harddrive activity on a Windows ME computer.
When it is inactive for a few hours it will become VERY SLOW (takes forever to refresh screen) the harddrive light is blinking, and clicking (as if defraging). I’ve opened uptask monitor and turned off everything extra but still SLOW, and blinking harddrive. If I force a reboot, all is fine, back to normal…till it sits a long time, like overnight, then back to it again.
Sam