Fragmentation happens to a hard disk over time as you save, change, or delete files. The changes that you save to a file are often stored at a location on the hard disk that's different from the original file. Additional changes are saved to even more locations. Over time, both the file and the hard disk itself become fragmented, and your computer slows down as it has to look in many different places to open a file.
Disk defragmentation is a technique to rearrange files stored on hard disk to occupy contiguous storage locations. The purpose is to optimize the time it takes to read and write files to/from the disk by minimizing head travel time and maximizing the transfer rate. Defragmentation of hard disk improves the access speed and also improves the system startup times.
Disk Defragmenter is a tool that rearranges the data on your hard disk and reunites fragmented files so your computer can run more efficiently. In Windows Vista, you can run Disk Defragmenter manually or you can schedule it so you don't have to remember to run it.
To run Disk Defragmenter, use following steps:
1) Open Disk Defragmenter by clicking on the “Start -> All Programs -> Accessories -> System Tools -> Disk Defragmenter”. If you are not the administrator on the system the you will be prompted to enter administrator credentials to proceed.
2) Click on “Defragment Now”.
Disk Defragmenter might take from several minutes to a few hours to finish, depending on the size and degree of fragmentation of your hard disk. You can still use your computer during the defragmentation process.