Fanless system?

I'm looking to build or buy a small fanless, headless PC to run Arch Linux for a file server and a few network services. I don't think needs a lot of grunt, but it needs to be silent as it'll be running constantly in a room where I also sleep and work. What are my options? Budget is roughly 400 euro, but I'd prefer it as cheap as possible. I've got a SATA HD (full size) I'm planning to shove in it.