Okay, you are legally required to purchase an additional copy of Microsoft Windows XP - as Acolyte says, if you have the copy in question already installed and operating on a machine, you can not use it on another machine. Also, there are further complications such as the copy that is on your current machine may only be allowed on that machine since it was shipped with Windows.
When building a new machine, one usually needs to purchase: A case (Upright Tower vs Flat Desktop) A PSU (power supply - often comes with the case) A motherboard (holds the CPU, Memory and handlers/sockets for perepherils) A CPU (The processor) Memory Hard Disk Drive CDROM (Optical) Drive Floppy Disk Drive
The CPU is dependent on what Motherboard you have chosen. The Motherboard has to fit inside the case that you purchase. The Memory is dependent on the Motherboard and sometimes the CPU.
Cables are important - when you purchase the Motherboard, it may well come with all the cables you need to plug in and connect all your internal perepherils to the Motherboard. E.g. SATA, IDE, Floppy, USB, Firewire, addition USB cables.
That's off the top of my head, any more queries just ask! Not had a search for any online tutorials yet but you should be able to find some easily. Something for me to try later for you!
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