Indeed a file server requires almost 0 CPU power. Pretty much any computer, even a 10 year old one, can be a decent file server. What it does require, is a bit of I/O (input / output) bandwidth (or throughput), because all that a file server does is take files from the hard disk and send them over the network, or the other way around, which is pure I/O.
Your PC will make a fine file server. If it\'s running Windows, right-click on the drives and Share them with Everyone. Linux will find them. It does mean everyone in your home can see all the files though, so you might want instead to only share a folder (same procedure, only right-click n the folder, not the whole drive). You can also password-protect your shares, allow some/all people to write/delete stuff on them... it\'s a bit more complicated, but you can google up tutorials. Linux can share too, of course.
Actually, a Pi would make an OK file server too, as long as there aren\'t too many client PCs (2 is fine). You\'d need the disk to be in USB enclosures though, the Pi doesn\'t have IDE nor SATA ports. It would be a nice project, and would save on electricity :-p
If what you want to share are films or music, there\'s a relatively new standard called DLNA or UPnP that makes it a bit easier to navigate and watch content. People is these forums are hard at work on making sure the Pi can act as a UPnP/DLNA server or client soon after release. My phone does both, so it should be easy ^^ A very well regarded Windows DLNA server is TVersity, and it\'s free.
PS: as always, Wikipedia is a good place to start learning: