shmerl: Most people won't install anything special. Installation of a new OS is not something a majority of consumers ever do. That's exactly what MS use to preserve their monopoly, since many hardware vendors don't sell their computers with anything besides Windows. A prebuilt console with Linux preinstaleld and sold in big numbers will shift the balance a lot.
Johnathanamz: Watch I bet you most people will install Windows on the Steamboxes.
Before you made that bet, you should have considered how much Windows OS costs when bought separately on an existing PC (be it Steambox, or not). That alone will probably make sure most people will certainly not buy Windows from store in order to install it separately. Most people get OEM Windows (usually preloaded on a new PC) because it is considerably cheaper then.
Unless, of course, Microsoft completely changes their pricing for desktop Windows OS.
Add to that that for most people installing Windows OS yourself is about as scary as installing Linux from scratch yourself, even more so if you intend to install it on a system which came preloaded with some other OS (e.g. SteamOS).