If you just want to use an old PC as a LINUX File Server then the best distro I've seen is called SME Server and is available from www.contribs.org.
It can provide firewall services if you have a PC with 2 network cards, and/or can run as a LAN Server and provide a Webmail services, Local Websites, iBay Storage Areas. It also provides local LDAP, SAMBA, Appletalk, DHCP, SSH, FTP and HTTP services.
The current version 6.1 is a bit old but runs perfectly, very stable (my box has run non-stop for over 6 months without a hiccup) based around RedHat 7. You control the whole thing remotely through a web-based administrator. There are plenty of add-on modules provided by other users to install anti-virus, spam filters, mambo, zope and plone etc - very kewl and easy for a beginner - just need to understand how to login using ssh and run a shell script.
The download ISO is about 350Mb and you can have the basic system up and running in under an hour. Stacks of help on the contribs.org website.
Check it out.
A Mac User Tries Linux Part 1