Let me first introduce the idea about disk fragmentation to unexperienced computer users. When you use a computer, permanent data are usually stored on disks. There are several different ways to handle data on disks. In order to manage data in a unified way, operating systems offer file systems that handle data in the form of files. Because file sizes change dynamically, storing files as linked lists is a good idea. However, there are still other ways to store files on disks, such as using NTFS, that is, Windows NT File System, which stores files using B-trees. Under operating systems that store files as linked lists, many unpredictable reading and writing operations results in such a fact: a file may be split into pieces. As disks are not great for random access, they slow down. Most other file systems are likewise. When a file is in pieces, we call each piece a file fragment.
Widely used disk defragmenters for Windows include: Norton Speed Disk, ExecSoft Diskeeper, Windows 95/98/Me Defrag, Windows 2000/XP Defrag, Golden Bow VOpt, and so on. Norton Speed Disk is a product included in Norton Utilities. Under Windows NT/2000/XP, many traditional programs in Norton Utilities are impossible to run, so Norton Speed Disk becomes one of the most valuable programs in Norton Utilities today (others include Norton Disk Doctor, WinDoctor, etc.). Norton Speed Disk and ExecSoft Diskeeper are excellent defragmenters. They not only do defragmentation very thoroughly, but also arranges files into a state that they can be accessed and defragmented in a shorter time afterwards. VOpt is a fast defragmentation tool, which defragments very fast but not very thoroughly. It leaves some minor fragments every time it finishes defragmentation. Thus, it does the job faster than other defragmenters. Windows Defragmenter in Windows 95/98/Me are derived from Speed Disk for MS-DOS version, so they are not very fast. Windows 2000 Defragmenter defrags faster but not as fast as Speed Disk. Anyway, Windows 2000 Defragmenter is good enough for general use. I suggest that a personal computer user should defrag his disks once every two months. No more deframentation is necessary. Of course a server should have its disks defragmented once a week.
As for Executive Software Diskeeper and Windows NT 4.0 disk defragmentation support, there is a story (quoted from "Inside Windows NT Disk Defragmenting" by Mark Russinovich):
The History of Disk Defragmenting on NT
In April 1995, Executive Software released the first defragmenter for NT: Diskeeper for NT 3.5 with Service Pack 1 (SP1) or Service Pack 2 (SP2). The company announced an update for NT 3.51 in July of the same year.
Because Microsoft did not design NT 3.5x's native file systems, FAT and NTFS, with disk defragmenting in mind, FAT and NTFS have no support for moving clusters around a disk. Executive Software purchased an NT source license from Microsoft to modify NT and the FAT and NTFS drivers to support disk defragmenting. As a result, Executive Software shipped its custom version of NT with Diskeeper for NT 3.51.
But as Diskeeper users upgraded NT 3.51 with SPs, they experienced problems. Installing an SP overwrote some of Diskeeper's files and left others alone, causing incompatibility problems for upgraded systems--and technical support problems for Microsoft and Executive Software.
As development of NT 4.0 began in 1995, Microsoft invited a Diskeeper developer to Redmond, Washington, to participate in the design and implementation of NT 4.0's defragmentation support. Basing Diskeeper on NT's built-in support let Executive Software avoid shipping custom versions of NT and meant Microsoft's technical support didn't have to troubleshoot non-standard versions of NT. One month before NT 4.0's public release, Executive Software offered a trial version of Diskeeper 2.0--the version for NT 4.0--on its Web site. (For Jonathan J. Chau's review of Diskeeper 2.0, see Lab Reports, "Diskeeper 2.0," April 1997.) Symantec has since entered the NT disk defragmenting market with its Norton Utilities Speed Disk, which also uses the NT 4.0 defragmentation support.