Disk Idle Optimizer 4.0
Disk Idle Optimizer (DIO) allows you to turn on and off automatic optimization functions that are built into Windows XP with Service pack 2.
Defrag your hard drive with your spare CPU cycles! Using the built in disk defragger in Windows XP (SP2 Required), this program senses when your computer is at an idle state and defrags your hard drive. As soon as you begin using your computer again, Disk Idle Optimizer will stop defragging, allowing you to use your PC to its fullest.
This program is freeware.
Requires Windows XP with SP2
Download: Disk Idle Optimizer 4.0