Over at the AVSforum, there is an interesting thread that includes instructions and a download link to allow you to turn a computer into a NAS using a version of Synology's diskstation software. The thread is eleven pages long and has many interesting posts. Overall, this approach does not seem feasible for any but advanced computer users. But, if you are interested in experimenting with NAS distributions, this is another approach:
Synology made NASs are one of the more popular home appliances available. Some models are ARM based others Intel_x86. However most are considered pricey especially when there are opensource alternatives readily available that accomplish what the QNAPs/Synology of the world do if not better. The files below will allow you to install Synology's OS, called DSM, onto a regular PC so long as the your hardware is supported. I also provided VMware and VirtualBox images if you dont have a PC lying around to test with but would still like to see whats its like to run a Synology DiskStation in your home environment. Again, to install on 'real' hardware please make sure your hardware/nic/sata/scsi chipset is listed below or IT WONT WORK. The virtual image will run on top of either VMWareWorkstation/ESXi or VirtualBox on any hardware you may have.