A pc case, preferably gutted of all but the bare bones systems. Examples are the power supply, the mother board (if it's a tower style server of an old pc) and it would be an excellent idea to take out the memory in the older pc.
buy a SATA internal card.
buy as many sata internal drives as you need
hook them up with thier power cables and connect them to your internal card.
replace your computer's old memory with no less than a gig of ram. the more ram the better in a server.
make sure you have a networking device, ethernet card, wi-fi card, etc.
then if you don't wish to use it for any other perpouse, you may not even have to use windows on it, if networked properly, the network will see it as a remote dive on the network.
