I think you'll find that the regular disc accessing is the system 'indexing' files in the backround so that it is permanently aware of what and where everything is. You could disable it in Windows 2000, but it's not a good idea really because when the system is asked to find a file it will then have to go and do it while you wait. Probably can disable it in XP too, but I haven't looked.
BUT. System idle process is a misnomer. It's not a process. It just means that the system isn't doing anything! 'System Idle' wouldn't confuse half as much would it?