Over time, as you delete and save files to a hard disk, the files become fragmented. This means that they are split into many non contiguous areas on the disk.One of the best ways to protect both your hard disk and the data on it is to periodically defragment the files on the disk. This serves two purposes. One is that by ensuring that all of the files are stored in contiguous sectors on the disk,head movement and drive wear and tear will be minimized.This has the added benefit of improving the speed which files will be retrieved from the drive by reducing the head thrashing that occurs every time a fragmented file is accessed.The second major benefit of a fragmented disk is that in the case of a disaster where FAT and root directory is severly damage, the data on the drive can usually be recovered very easily if the files are contiguous.
Note: Cleaning your registry and your temporary internet files folder is another way of optimizing system performance.