1) That PC box has to have an OS on it of some sort to serve files. If you already have Windows on it, just use that. If you don't, or want to move to Linux, then there are SCADS of sites to get you through that, but I'll warn you, it's not for the non-techie. (Yes, yes, before anyone screams at me, it's gotten much better, but considering that he's not even sure what Linux *is*...) redhat.org, debian.org are two big ones.
2) Once you have *an* OS on it, multiple drives *should* be immediately supported, but that's between you, your OS of choice, and those drives. On the Mac, it just works painlessly. Windows, pretty darned good unless it flakes out, and I honestly don't know what the state of the art is on Linux at the moment. Might have to muck with /dev, might not. I'm hoping not, but that'd depend on what GUI shell and tools you choose to install.
3) I don't know where you'd find the file sharing under your current system, particularly since I don't know what system it is.
Me, I'd put a basic Linux or Darwin installation on it, and call it good, but I'm more familiar with the guts of those systems than you may be.