We only have one computer here with windows 8, and it works fine. Though we have not done enough test to know for sure, but I would THINK that windows 8 and Vista would be all fine... but I'm a little unsure about vista, and I know that XP have some problems atm. But we might look into XP support further down the line.
Here is a quick overview of things we want to work on in the futre:
roadmap:http://steamcommunity.com/app/262390/discussions/0/558751813532134586/
XP support should be somewhere in there, though it's not that high priority as it's fairly few steam users who seem to be using it.
Well, that’s the breakdown on our OS support as far as I know for now at least. Hope that helped, or else some other users might be able to give you more info : )