In hard disks data stored randomly on the surface of the disk.de frangmentation is the process to arrange them in an order and free up space in the disk.Advantages of defragmentation is to utilize the free space and to increase the capacity of the drive as well as the speed.
Disk Defragmenter(defragmenting volumes that use the FAT, FAT32, or NTFS file system.) is a utility in Microsoft Windows designed to increase access speed by rearranging files stored on a disk to occupy contiguous storage locations.
Open My Computer > Right-click the local disk volume that you want to defragment, and then click Properties > On the Tools tab, click Defragment Now > Click Defragment.
Start Computer Management MMC (Compmgmt.msc) > Click Disk Defragmenter > Click the volume that you want to defragment > then click Defragment.
Start Disk Defragmenter MMC (Dfrg.msc) > Click the volume that you want to defragment > then click Defragment.
Windows Disk Defragmenter
Copyright (c) 2001 Microsoft Corp. and Executive Software International, Inc.
defrag <volume> [-a] [-f] [-v] [-?]
volume drive letter or mount point (d: or d:\vol\mountpoint)
-a Analyze only
-f Force defragmentation even if free space is low
-v Verbose output
-? Display this help text
Eg. defrag c: -f -v
Scheduling defragmentation using command ‘at’
at 07:00 /interactive /every:M,T,W,Th,F,S,Su “defrag c: -f”
for running it every day 07 AM
at 07:00 /interactive /next:Thursdy “defrag c: -f”
defrag it on next thursday 7 AM
for more informations about using at command type at/?