PC Tonic

A hard disk consists of small storage units called tracks. These tracks join to form a sector. When you save a file on your computer, it is first fragmented into smaller bits and every portion is stored in some random track. The next time you open that file, your operating system will collect all the fragments, join them and then open the file for you. By computer means, this takes a lot of time.
Disk defragmentation tools reduce the time to retrieve data by defragmenting it. Defragmenting the hard drives brings all the related data together and pushes the free space to one corner. So, all the file fragments are now closer to one another and can be retrieved faster. Even though the increase in speed is not noticeable, it is recommended to defragment your hard disk or drives (especially C:) periodically.
Windows has an inbuilt defragmenting tool, which can be accessed from Start > All Programs / Programs > Accessories > System Tools > Defragment. However, it’s unpopular due to it’s incredibly low speed. So I have mentioned some free third party defragmenting tools – you can download and use any one of these.

Recommended: AusLogics Disk Defrag

AusLogics Disk Defrag ~ AusLogics Software
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Available for: Windows Vista/XP/2003 Server/2000
Home | Download.com

IOBit SmartDefrag* ~ IOBit.com
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Available for: Windows Vista/XP/2000
Home | Download.com

Defraggler* ~ Piriform
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Available for: Windows Vista/2003 Server/XP/2000
Home | FileHippo

JkDefrag ~ JC Kessels
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Available for: Windows Vista/2003 Server/XP/2000
Home | FileHippo