Decide which distro you wanna use. If this machine will be just a server, sitting in a corner somewhere with no monitor or anything, a huge desktop distro like Mandrake or Red Hat isn't necessary.
Slackware and Debian are good server distros, or FreeBSD even. Debian has the best software management system of the Linux distros out there by far, but FreeBSD's is arguably better if you don't need to run Linux-only apps, hehe. Slackware is the 'lean and mean' one of the three. A basic install of it leaves nothing but what you need with the OS, allowing for a bit more customization.
For the actual file/print server, Samba is what you'll be using. Google "Samba How-To" and you'll turn up tons of info and instructions. It's pretty easy to set up
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