Based on the FreeBSD operating system (a Unix derivative), FreeNAS is a server operating system that offers lots of features, a very small footprint, and a can't-beat-it price (it's free).
Developed by an open-source community, FreeNAS is constantly evolving (with even nightly builds).
FreeNAS is more complicated to install and use than Microsoft's more feature-rich Windows Home Server, but people willing to navigate the sometimes confusing installation routine are rewarded with a robust network-attached storage device.
Choose your hardware
The hardware requirements for FreeNAS are pretty minimal: a motherboard with an x86 processor, 128MB of RAM, 32MB of free drive space (on a bootable drive, a CompactFlash card, or a USB key), a network card, and a BIOS that supports a bootable CD-ROM.
We installed FreeNAS on a tiny Via Artigo PC (a small, impressively power-efficient little system). As networked-attached storage this is an ideal option, as you should consider how many Watts your old PC will draw in 24/7 operation before putting it into service as a NAS device. During FreeNAS installation, you'll also need a monitor, a keyboard, and a CD-ROM drive, but afterward you won't need them for your NAS box.
To begin the process, we downloaded the FreeNAS ISO disc image from freenas.org (version 0.686.3, revision 3011, was the current, most stable one available at the time of this writing). We also downloaded a PDF of the installation guide, which is a must for navigating the somewhat unintuitive process.
We burned the ISO image to a CD, booted our intended NAS box from it, and began the speedy process of installation. We selected the option to create two partitions on our hard drive, one for FreeNAS and one for data. Then we worked our way through a series of tasks that included configuring the network interface and setting the box's IP address. Once those steps were done, we could access the FreeNAS box via a web browser from another PC on my network.
Doing just that, we instructed FreeNAS to mount the second partition on our drive, and then initiated CIFS (Common Internet File System), the protocol that lets Windows PCs connect to the NAS. (For a Linux PC, you would use the NFS - Network File System - protocol). FreeNAS also supports various flavors of RAID, but the development team suggests configuring each disk individually - to ensure they work well - before establishing a RAID setup.
You could stop here and have a very useful device, but FreeNAS has numerous other features worth exploring, including RSYNCD, a network utility for incremental backups over the network; Unison, a file-syncing tool; and FTP, for easy file transfers. Plus, we like the freeware version of SyncBack from 2BrightSparks, a file-backup and synchronisation tool - and it works like a charm with a FreeNAS box.
We're impressed with FreeNAS. Windows Home Server has more media-friendly capabilities such as Xbox 360 connectivity, but if you are looking to set up a basic NAS box using your existing hardware, for free, FreeNAS is well worth a spin.