So taking what xcleric42 said, if you have a hard drive with lots of storage space, and it isn't full, meaning you have a good couple of Gigs free, you won't really need to defrag. Once your drive is close to capacity, and you find that file or application access is slow, then you should defragment your hard drive. A good way to keep ahead of this is by having 2 seperate drives in your PC, one for storing your files on, and one running your applications.