There's also a simple-to-use scheduler, so that you can schedule your defragments at regular intervals. You can also customize the defragmentation in many ways, such as not defragging when your PC is on battery power.
One particularly nice feature is Auto Defrag, which defragments your hard disk only when you computer is idle, so that you don't interrupt or slow down any work. You can also customize Auto Defrag by telling it to pause when your system exceeds a certain percentage of resource usage, for example.
Note that by default, this program installs the Yahoo Toolbar, so if you don't want it installed, uncheck the box when you install the program.