JkDefrag – Optimize you harddisk for free

Free (released under the GNU General Public License) disk defragment and optimize utility for Windows 2000/2003/XP/Vista/X64. Completely automatic and very easy to use, fast, low overhead, with two optimization strategies, and can handle floppies and USB disks/sticks. Included are a Windows version, a command line version (for scheduling by the task scheduler or for use from administrator scripts), a screensaver version, a DLL library (for use from programming languages), and versions for Windows X64.

jkDefrag is based on the standard defragmentation API by Microsoft, a system library that is included in Windows 2000, 2003, XP, and Vista. All defragmenters are based on this API, including commercial defragmenters. JkDefrag is therefore very solid and there is no risk of losing data. You can stop the program at any time, it will finish the current file in the background. If your disks use NTFS then you're even safe when the computer crashes in the middle of defragging. Nevertheless, it's still a good idea to backup before defragmenting, just like with other defragmenters, because the heavy use of the harddisk may trigger a hardware fault.