Because it is the place where you permanently store your applications and, more importantly, the data files you create with those applications, your hard disk requires special attention to keep it operating at peak efficiency. Disk scans for "lost" files and bad sectors will prevent most disk problems before they occur, while running a disk defragmentation utility will improve the performance of your system.
Every day, you create new files, delete unwanted ones, and write updated versions of current files to your computer's hard disk. Because of the way Windows and the Mac OS assign disk space to files, your hard disk can become fragmented over time (i.e., your files get placed in pieces all over the hard disk, because there is no contiguous space large enough to hold them). A fragmented hard disk slows disk access and makes it harder to recover from disk errors.
To defragment a Windows 95/98/ME/XP/Vista/7 hard disk, use the Disk Defragmenter program in Programs/Accessories/System Tools. On a Mac, use a utility such as the Speed Disk tool that comes with Norton Utilities for the Macintosh. You should defrag your hard disk every six months. You should also defrag it after you have created or deleted a lot of files in a short span of time.
Alternatively, you could download a free disk defragmenter program like Auslogics Disk Defrag. A very good, and very fast disk defrag utility.
Occasionally, a storage area called a sector on your hard disk goes bad. A utility called a disk scanner detects such "hard" errors and keeps a table of such sectors so that your operating system doesn't try to use them to store files. In addition, a disk scanner detects "soft" errors, where the operating system has lost track of pieces of one or more files. ScanDisk (found in Programs/Accessories/System tools) is Windows 95/98/ME/XP/Vista/7 built-in disk scanner, and it detects both hard and soft errors. On the Mac, you can use the Disk First Aid program that comes with your system or the more powerful Disk Doctor utility included in the ubiquitous Norton Utilities for Macintosh. You should run a hard disk scan at least every month. In addition, if your computer crashes or you accidentally shut down without going through the proper shutdown procedure, run a disk scan as soon as you reboot.