so i've got an old PC kicking about and i want to set it up as a file server, what OS would you recommend for that? i assume some flavour of linux would be good, but i have absolutely zero knowledge/experience of linux at all (i'm absolutely willing to learn mind you, i'm not going to freak out because things are omg different from windows) so i dunno where to start.

freebsd. pc-bsd if a normal unix shell scares you.

If you don't feel like arsing around the command line too much, there are some good prebuilt distros for file sharing (i.e. http://www.freenas.org/ )

Don't listen to guys above. Install Fedora or Ubunty and try to set up a simple server. Use google, mans and your head and you'll get what you want.
If you want it "just works" and have no time to dig up linux use Windows.

How old your PC is?

Why should he not listen to them? Anyways, maybe Ubuntu could be too much for the old computer, it has too many unwanted applications that probably he would never use.
Specs would be good. Besides, we don't know why he wants to share files, or how. Try to be a little more specific.
I would be afraid to use windows for a file server, too many risks. Of course there are risks in linux, but those can be easily fixed with a few minutes and googleing.