What operating system can I use to turn my old computer into a home server?

"FreeNAS or OpenFiler Both are designed for sharing files and both are free.

I use FreeNAS, it is easy for me and I like the features. I say give them both a try."

I run a web/file server on a PIII 450 with 640mb of ram. For a 'pure' fileserver, freenas is nice, else going for a cli only install of ubuntu works well. Hardwarewise, you'd need at least one network port, naturally, and you could set up the box headless, doing admin tasks via openssh. I'm using less than 100 mb of ram right now with a file/web server based off ubuntu so... yeah, it would probably work