Custom built systems give you the option of the important components that go into the unit giving you considerable say in how your PC is made. For example, you may choose from three or four different processor speeds, select the amount of memory you want, and perhaps even the brand of hard disk your PC will use. The end result: you get a powerful, well-configured machine that only has what you want on it - no unwanted software bogging the computer down, no slow hardware that you can't find drivers for, no hassles. Just turn it on and go.