A complete system backup, such as a disk image, will help you recover quickly from a drive failure or other catastrophe, but it adds the expense of a second hard drive (or potentially extensive disc swapping if you use your optical drive). The best time to create a drive image is immediately after you reinstall Windows and get your applications running again. An image containing a patched copy of Windows and all your favorite programs configured the way you want them is a very useful thing to have at hand.