Since the days of Windows 9x Microsoft has been providing a tool which is very effective in reducing the read/write time of the hard drive. This tool is the Disk Defragmenting tool known as Disk Defragmenter(defrag.exe). With the course of time and usage the data on the hard drive gets scattered. This happens because we create files, we save files, and we keep on deleting files. File deletion results in empty clusters in the hard drive and this empty space is again used for saving the new files. Let us understand fragmentation by an example. Lets assume there are 10 clusters on a hard drive and these are being used by three files A, B, and C. File A is occupying clusters 1-3, file B is written on clusters 4-5 and similarly file is C is on 6-8 and clusters 9 and 10 are empty. Now if we delete file B the clusters 4 and 5 becomes empty. Further if we have to save a new file (file D) which needs 4 clusters to get saved then the operating system will save half of the file D on clusters 4 and 5 and rest on 9 and 10. This makes the file D fragmented and whenever you will access the file D it will be accessed from two different locations and thus it increases the read time. The Disk Defragmenter fixes this by rearranging all the files in sequential clusters, and reduces the read/write time on the hard drive thus resulting in high operational speed of your computer. I still remember the graphical interface of Disk Defragmenter which was there in Windows 98 it actually displayed all the clusters of hard drive as blocks and displayed the real-time arrangement done by the defragmenter.
Like all the predecessors Windows 7 also has Disk Defragmenter in it, which keeps on rearranging the files on real-time basis in the background. But if we want to run it manually then we can do the same by going to Start menu and opening the All programs list. In All Programs list open the Accessories sub menu and then the System Tools. In System Tools menu you will find the Disk Defragmenter tool. Running this utility would require administrator privileges and only after that the utilities window will come up. Once the Window is up just click on Defragment now and it will do all the re-arranging itself. Unlike Windows 9x or Windows 2K the Defragmenter of Windows 7 does not show the defragment happening and does it silently in the background.
Disk Defragment does not move all the files, and the files which are address specific (like booting files and boot code data) stays untouched. It is wise to run disk cleanup before running the defragmenter because if you delete unused files from a defragmented disk then it will again create fragments of free space. Disk cleanup tool can also be found at the same location, which is Start menu, then All Programs, then Accessories, and then System Tools.