It's just a shared folder. If you don't feel comfy with linux, I'd buy a disk or pair of disks, and install them (replacing the old disk). Reinstall XP and activate. Then, make a folder and share it. That's all. XP allows something like 5 people to connect to the server and share files.
Also, remove any cards not related to serving files. Cards take 1 or 2 watts, and graphics cards use a lot more.
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