Defragmenting speeds up computer

When a file is formed on a computer disk, it may not occupy on it in a contiguous order. Moreover files that are deleted produce in their place a gap on the disk between the existing adjacent files. These files are called the fragmented files. These files slow down the speed of processing by computer to read or write a file. So, they require Defragmentation. With this defragmentation process the contents of the disk are organized close together in contiguity. This makes faster the computer's processing of files apart from creating free space by compacting the files.
Defragment (in Windows XP OS):
Start --> All Programs --> Accessories --> System Tools --> Disk Defragmenter --> select Disk --> click Defragment tab.

Click and read more about Defragmentation here:
Defragmenting
How to Defragment files?