Theoretically you could use any.
Windows would probably be the most stressful because you would want to stop all of the automatic "phone home to Uncle Bill", Windows Update, Windows needs to reboot, Installing updates, your computer is at risk, and all of the other background rubbish. If the Windows computer was carefully and thoroughly configured for the job, then it should be possible. Personally I'd rather attempt it with XP than Vista, but that's mostly because of my familiarity with it.
I don't know about Macs, so I'll leave that one for you.
My first choice would be Linux. If the thing is going to be running non-stop for the best part of a year then I would spend some time building it from scratch.
How much disk space
