Why do defragmentation?

One of the reasons of slow or unstable performance of your PC is cluttered hard disk. Defragmentation is a process that reduces the amount of fragmentation in file systems. It does this by physically reorganizing the contents of the disk to store the pieces of each file close together and contiguously. It also creates larger regions of free space. Some defragmenters also try to keep smaller files within a single directory together, as they are often accessed in sequence.

Defragmentation of the hard disk improves system performance, stability and speed. In windows you can right click the hard drive.
You can google and find free utilities to defragment or you can also right click your drive , go to Properties.. go to Tool ..and select and you will see the process of defragment.