If it's just a file server (you can make it a print server as well if you wanted) then XP will work fine. All you need to do is setup shares. I was using XP for a long time as my file server OS. As my friend says, you can find XP product keys in your couch.
Heck, if you need one I have an unused XP Pro key I don't need.
I decided to upgrade it to Windows 7 awhile back because I had an extra key. Now I picked up an actual server system and I'm in the process of setting up a domain with Server 2012.