Parts that are required for a computer to run and work properly are;
A motherboard (if you have an onboard graphics card, ignore the Graphics card, unless you're after a more powerful computer) Memory (Also known as RAM) A Processor A Hard Drive A graphics card A CD Drive (To install your operating system) A Power Supply
Note: If you are installing Windows XP on a SATA hard drive, you'll need a floppy disk drive with the "Hard Drive" drivers, so to speak.