What you actually defragment is your computer's hard drive. In time, because you copy, move, create and remove files and folders, as well as install and uninstall programs, the way data is saved on your hard drive physical tracks becomes fragmented. This generates unefficiencies on its operation. Defragmentation is a very simple procedure: -In Windows XP: Right click your "My Computer Icon" and select Explore. You will see all your drive units. Usually (99.9% of the times) your hard drive is C: Just right click on it on select Properties. You will see some Tabs on the next window. Select the "Tools" tab, and you will see a Defragmention option. Click the "Defragment Now" button. Click Defragment when the Defragmentation Window pops up. -Another way (that works for all Windows versions): On your desktop, click the Start Button (lower left side corner). Select "All Programs", on the list that appears, go to the "Accesories" sub folder, and then to the "System Tools" sub folder. Choose the "Disk Defragmenter". Click Defragment when the Defragmentation Window pops up. Because the disk is relocating data, and with the big size drives nowadays, it will probably take some hours, so I recommend you leave the PC running the process without doing anything else. A good time, is when you go to sleep, so next morning the process will be finished for sure. I used to stare for long periods of time to the defragmentation status bar, you can see how your disk is being "organized". You can do it the first time to understand better how the process works. Hope the instructions are clear enough.

I just wanted to add a few comments on this topic. First, defrag is something you want to do regularly, but not on its own. The first thing you should do is check the drive for errors, or scandisk as it's known on older versions of Windows. The reason for this is that defrag is going to move parts of files all over the place on the hard drive, so you want to be sure the drive is still OK beforehand. Then, do the defrag and finally, (especially if you are cautious like me) run scandisk again, just to be sure all the files got put back together in the right way. Yes, I am being anal about this, but wouldn't you rather know something is wrong right away? Or months down the road? Second, defrag doesn't have to take hours to complete. In fact, if you make defrag part of your weekly (or even monthly) routine, there won't be as much to defrag and it will take far less time. In fact, if you defrag often, the program will likely suggest a defrag is not needed. Ignore that advice and do it anyway. It will take less time and keep your files in one piece like they were intended.

Here is a step by step process on how to do it. Its simple - http://windows.about.com/od/troubleshooting/ss/defrag.htm

Go to Programs: Accessories: System Tools: Disk Defragmenter and click the "defragment" button.

Start menu All programs Accessories System tools Disk defragmenter

open my computer right click your disk, click properties click tools tab click defragment, click start if you have under 15% free space, it complains BTW, which OS are you using?

how do i defragmentize my computer?