From J.C. Kessels
MyDefrag is a disk defragmenter and optimizer (a maintenance utility to make your harddisk faster) for Windows 2000, 2003, XP, Vista, 2008, Win7, and for X64. It is freeware, no time limit, fully functional, no advertisements. Fast, low overhead, with many optimization strategies, can handle floppies, USB disks, memory sticks, and anything else that looks like a disk to Windows. Included are a set of easy to use scripts for endusers, a scripting engine for demanding users, a screensaver, and a combined Windows plus commandline version that can be scheduled by the Windows task scheduler or for use from administrator scripts.
MyDefrag is extremely solid because it is based on the standard defragmentation API by Microsoft, a system library that is included in Windows 2000, 2003, XP, Vista, 2008, and Win7. Most defragmenters are based on this API, including the free defragmenter that comes with Windows and many commercial defragmenters. Basically all MyDefrag does is send "move this file to that location" commands to the API. The API is part of the operating system and can safely move almost any file on the disk, without risk and while the computer is in full use.
Imagine a book split into several parts, some pages are over here, other pages in another