If your computer is slow, do you think about defragging it? Do you even know what defragging means? Defragging or 
putting your computer through the defragmentation process allows your computer to access your most-used files faster. Essentially, the computer will put your everyday files and applications, such as e-mail and Microsoft Word. physically to the front of your hard drive where it is faster for the CPU to read and access that store info. Those programs and data, i.e. scheduled tasks and control panel, that you hardly use will be re-allocated to the back of your hard drive. In sum, the defrag utility that Windows contains by default allows your computer read your files faster.
Please note that if your computer is running slowly, chances are defragging might not help. A slow computer could mean possible viruses, spyware, or a failing hard drive among other things. If your hard drive is failing, defragging could only further exacerbate this damage and should be avoided. The best way to find out what to do with your computer is to get it diagnosed by a computer service technician.