The disk package should have an activation code included, if not, it's pirated. Also, once the game is activated on Steam, it doesn't need to be reactivated again.

So... even if you "lost" the disk, the game should be coded to your account and all you need to do is Just download and go. Anything outside of those, including trying to run the game without Steam is well.... pirated. Boozad is right on point here.