DEFRAG

Reorganizes the files on a disk to optimize disk performance. Do 
not use this command when Windows is running.

Note: The MS-DOS help said: Do not use this command when 
Windows is running. This means do not use DEFRAG.EXE for MS-DOS 
when Windows is running. Of course you can use DEFRAG for 
Windows when Windows is running. In Windows NT, there is not 
such a command DEFRAG. DEFRAG.EXE for MS-DOS was bought by 
Microsoft from Symantec, developed by Peter Norton group. This 
tools was originally included in Norton Utilities for MS-DOS 
as SPEEDISK (Speed Disk). After Microsoft got its DEFRAG.EXE, 
it remains in Windows up to now. DEFRAG for Windows in Windows 
95/98 is quite slow. DEFRAG for Windows Me is better. Speed Disk 
in Norton Utilities 4.0 for Windows is very fast. Peter Norton 
and his colleagues must have invented a new technology for 
disk defragmentation. So Norton Utilities is popular because 
of Speed Disk.

For more information, please refer to MS-DOS Help or Windows NT 
command line command help.