is a process that rearranges and reassembles your files on your hard disk. Over time, Windows will slowly fragment the files on the hard drive and store parts of each file in random physical locations. As this happens, the files will take longer and longer to access so deframentation becomes necessary to improve performance. See the The Importance of Disk Defragmentation
The length of time it takes to complete deframentation depends on your operating system, size of the hard drive, number of files to defrag and defragmentation utility used to perform the task but it can take several hours. Once you start the defrag process, you should leave the computer alone. Defragmenting utilizes a lot of system resources and will slow the computer. Although the process can be paused or stopped, it is best just leave the computer alone for however long it takes. There are third party utilities
that work much faster than the built-in Windows Disk Defragmenter
. The Disk Defragmenter screen allows you two options - Analyze and Defragment. Analyze will let you know whether the computer thinks it is time to defrag the hard drive and Defragment begins the process immediately.
The main thing that keeps defrag from running successfully is having other programs running in the background including your screensaver and anti-virus program. While using the Defrag utility you may receive a message every few minutes saying your computer is currently running other processes that cause the Defrag to restart. Since Defrag changes the bits of data on your hard drive, it does not allow other programs to run at the same time. Turn off the former by right-clicking the Desktop, choose Properties > Screensaver tab and choose None. If after turning off all running programs, you still find that the defrag routine is constantly interrupted, you can defrag from safe mode
. Also read How to Defrag: Prepare Your Computer for Defragmentation
users should refer to Understanding Windows Vista Disk Defragmenting Utility