Steam hardware survey

[Warhound]

C/P from a steam moderator :

Correct, only a certain number of Steam accounts are selected for the hardware survey each time the survey comes around.

It's on an "opt-in" basis rather than "opt-out. No information is sent until you click through the survey window to submit the information. If you do not want to participate, you simply click cancel and no information is sent.

So if you don't want to participate they don't know what hardware or software you are running.