Software for NAS box

[#1]

Ive just rebuilt an asus terminator (model K7 DDR) which i intend to use as a NAS box due to its perfect size and its quite adequate specs and was wondering whats best to run on it
Its currently got a cut down install of xp pro which i believe will do the job just fine but is there a better alternative out there ?
Ive been looking at and quite like the look of FreeNAS as it looks easy to setup and i want something i can just install and leave to do its job 
does anyone use it or have any views on it or any other possible (preferably free) alternatives

[#2]

I've used Debian for mine, it's stable and works without hassle and a fresh install to working secure NAS box is only about half an hour, 45 minutes. Having said that I'm consdering a change to ubuntu server, not that there's a great deal of difference. I'd say go for linux, it runs for weeks without you even needing to log on to the box, but that's just a personal thing.
Hope that helps,

[#3]

I agree 100%, definitely a Linux kernel, and I'd go for Ubuntu.

[#4]

Right then thank you guys that is very helpful indeed and fairly conclusive ubuntu server it is
Coincidently enough Peter i was toying with the idea of using it as a mail server as well so ubuntu
server should work out ok
So far over the years ive only messed about with various flavours of linux always going back to M$
purely because of its ease of use in everyday computing but im always willing to jump in and im also a
quick learner so hey ho here we go
Still i know who to ask should i come across a problem 

Ta muchly