MyDefrag is a disk defragmenter and optimizer, that is a maintenance utility to make your hard disk faste). 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 end-users, a scripting engine for demanding users, a screensaver, and a combined Windows plus command line 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 big library with lots of books, spread out all over the building and not sorted whatsoever. There is an index telling you exactly where every book is, but you will have to walk a lot when you need several books. This is exactly what happens on your hard disk, the files that belong to an application can be all over the place, anywhere on the hard disk. Optimisation will bring all the files together in one place, leaving the rest of the hard disk empty, and will sort the files, for example alphabetically.