Even though you have configured your Windows XP system to turn off the hard disks after x minutes of inactivity, the disks either never turn off, or they turn off many minutes later?
When Windows XP first starts, many system tasks are invoked to 'maintain' your system. These tasks may take 30 minutes or more, and they include hard drive activity. You disks won't turn off until this 'maintenance' is complete.
Additionally, the following activities will reset the idle timer:
Third-party services (antivirus, etc..). Windows Update checking the system state. Network detection (DHCP). Paging operations. Event logging. Scheduled tasks. The loading of services or drivers.