Well, I didn't know that I could use the backup tool in Steam and I didn't use it before reseting my Windows. I don't want to download The Witcher 3, neither lose my saved games, but I couldn't get Steam to recognize the files I copied from the folder /Steamapps/Common. I copied the folder back there and clicked in Intall, although it started downloading a new game. I really don't wanna download all 50GB of game again, considering I have a 10MB internet :( Anyone can help?
You can declare your old steam folder as a library. This should immediately make Witcher 3 playable again after a quick data integrity check.
Go to the Settings - Downloads. Click Steam Library Folders, then select your old steam folder(s).
I reinstalled Windows just a week ago. Set my old steam folder as a library and all my games worked again.
What Dulkan said is correct, however it happenned to me once and even that did not work. (Maybe they fixed it now)
What I did at the time was simply put everything in place and launch the .exe as it will automatically 'connect' with Steam.