[ergo14]

Thats data that is automaticly gathered by steam - it's not a "survey" where you can pick OS.

[ksk]

Is it opt-in or opt-out?

Edit: Nvm found my answer

http://store.steampowered.com/hwsurvey

Its optional. Also other people have mentioned that steam randomly chooses users to take part in the survey and then asks them if they want to participate.