When Windows Vista came out, a lot of people were actually very interested in a new version of Windows. After all, XP had been used for quite some time, had several quirks (security-wise), and so there was a lot of interest. Of course it quickly backfired, when users noticed all the quirks with Vista.
When Windows 7 came out, it really did influence PC sales. Users were enthusiastic about 7, it seemed that Microsoft finally "got it" and focused on providing a great experience both for home-users and business-users alike. It seemed that Windows 7 was designed according to users demands and wishes.
With Metro and Windows 8, it's the opposite. Even the enthusiasts say "It's not as bad as you think once you get used to it." What a horrible way of praising a new product - it's not as bad as you think.
Windows 8 has completely failed to attract end-consumes. Most end users find the interface useless and cumbersome. None of my non-techie friends would every buy a new computer to get Windows 8. And even my nerd-friends shy away from Windows 8.
So all in all: Tablets and smartphones eat aways desktop sales, but Windows 8 has rather accelerated this process than slowing it down.