You can defrag your hard disk with Norton Utilities, but I wouldn't recommend it. To understand why, consider two things: (1) What the word "fragmented" means, and (2) the operating system itself.
There are two kinds of fragmentation: file fragmentation and volume fragmentation. File fragmentation refers to what happens when a file gets broken into lots of tiny pieces so that every little nook and cranny of the hard disk can get used.
Volume fragmentation (the kind that Norton looooves to point out) just means that all of the files aren't crammed together in one (mostly) contiguous part of the hard disk. File fragmentation can be bad--volume fragmentation: not really.
In Classic Mac file systems (HFS and HFS+) and DOS/Windows file systems (FAT, FAT32, NTFS), an "optimized" disk means that not only are files themselves contiguous, but all those files are jammed together in a tidy block. It seems efficient and neat, however... from the second that a disk has been defragged, it becomes more and more inefficient. All that packing together of data means that the moment a temp file is deleted, part of your business proposal is probably going to get stuffed in it's place--instant fragmentation.
In UNIX, however (like MacOS X's underpinnings), typical filesystems use high degrees of _volume_ fragmentation to combat file fragmentation. Anybody who has had some sectors go bad on a hard drive knows that lots of file fragments mean the possibility of a much greater number of corrupted files.
In short (oops, too late). Defragging in the UNIX (and now MacOS) world is more likely to do harm than good.