You can use any parts in your computer as long as they fit. Once you have the right parts, you can build it in a short time. By "right " i mean the actual part, like ram, Hard drive etc
My computer didn't cost me anything to make, because i was able to get parts from family members, and scavenging.
Throughout the instructable, there will be links. I give a basic overview, and the links are for a more in depth look at the part.
I also give a relate each part to parts of the body if i can.
Also, keep In mind I have never taken any classes about computers, except for the typing class at my school, and that this Instructable is the most research I've ever done about computers, so try to keep harsh comments to a minimum please! It would help more if you tell me what I've missed without being sarcastic
Step 1: The Materials
-a Hard Drive
-a Processor with fan and heatsink
-256mb or more of ram
-a Power Supply Unit-(PSU)
-cables to connect it all
But to add form to function you need, in addition to that above-
-a case - AT, ATX, BTX, or LPX -a modem
-an ethernet card
and if you want it to be even better-
-a graphics card
-a sound card
-another hard drive
Step 2: The Motherboard
More Here http://en.wikipedia.org/wiki/Motherboard