Ars Technica walks through building a Windows NAS:
At this point, I had a couple of options. I could spend more money on a better, faster NAS, one that wouldn’t disappoint me with its performance. Or… I could go ahead and build my own, which would give me the flexibility to build basically whatever box I wanted. Inspired by our recent articles on building a living room gaming PC and a DIY router, I decided to take the more Ars option.
Source: The ins and outs of planning and building your own home NAS | Ars Technica