Yes. I bought it for my grandson and he has a steam account. He used it with his gaming computer on a game he has through a steam account. It worked just fine. No problems.
Yes, I used this controller for 5 years with windows 7. There are two driver modes on this controller (changed via a switch on the back), D and X. Both work, but for games with XBox 360 compatibility, use X.
Yes there is Xbox wireless controller for windows including USB adapter and you can get Sony controller too with adapter. Please call the store to check stock availability or you can order it online from bestbuy.com