HDD runs all the time

Ihave an AMD K2 500 MHz compute where I am running Windows ME. for some unknown reason, which I have not been able to find, the HDD/s run all the time while the computer is left unattended. I have tried selective start-up to try to isolate the problem and have failed. While sitting at my XP computer alongside I can see the red light for the HDD on and hear the HDD running while the ME computer is un-attneded.As soon as I pres a key to open the computer it stops. But soon after the HDD starts running again. I have a RAM optimizer program but when I close this off the HDD still runs.