Over time and with regular use, files on your computer's hard drive break down and become fragmented. This will cause your system to run slower and experience processing issues. Defragmenting your computer will generally improves retrieval time & overall performance.
A computer program must access different files on your hard drive everytime it runs. If those files are spread out on opposite ends of your hard drive instead of being organized neatly, as they are supposed to be, a computer will have to work even harder and take more time to access the information needed. In rare cases, severe defragmenting on a hard drive may even cause a program to stop running entirely.

Defragmenting, also known as "defragging," reorganizes your hard drive by putting pieces of related data back together (like a puzzle) so that files are organized. As a result, your computer system can access files more efficiently and will speed up its processing ability. By efficiently organizing your files and folders, defragmenting will also leave more free space on your hard drive and in a large chunk.This will allow new files to be saved in an organized fashion, and by doing so, reducing the likelyhood of future fragmentation.
As the overall size of files stored on the hard drive increases, defragmenting your computer regularly may even increase its life-span. When you consider how much harder a system needs to work in order to collect fragmented information across larger and larger groupings of stored data, it seems likely that defragmenting can help your hard drive (and your processor) last
significantly longer.

Although some computer experts argue that today's operating systems are efficient enough to eliminate the need for defragmenting altogether (I disagree), it is generally recommended that the average computer users defrag their systems on a regular basis.
If you are an average user, you should consider defragging your computer at least twice a month. If you use your computer heavily, defragging will not only speed your computer but extend its life.
All computer systems come with some type of defragmenter tool, commonly found under the "System Tools" option in Windows-based Operating Sytem. These tools typically come with an analysis feature that will actually tell you if your computer need defragging or not. I recommend scheduling a defrag once a week just so when it is time for it, the computer has to work less when organizing the fragmented data. It is a lot like cleaning. Why wait until the house is a pig sty before organizing and cleaning it? The good news it that this is much easier. You simply schedule it if your computer allows for that, or click and step away until it is done. You can use your computer while a defrag is running, but this makes it harder on the computer and I would not recommend it.

It is my opinion that the Windows Vista defrag tool is crap. This being the case, I have a recommendation as an alternative you are so luck to be a Vista user. This program is also good for any Windows or Linux based operating system, but if you have Vista, this is a MUST! AUSLogics Disc Defrag has all that you need and more to be a power house defrag tool. It also has a built in and easy to use scheduler for those who are too lazy to remember to defrag like me. Here is a download link:
http://www.auslogics.com/en/software/disk-defrag/download