TestBot testing reveals that SM_MEDIACENTER is also set to 1 on Windows Vista and 7 (which both have Media Center included in the pricier editions) but not Windows 8 and later (which don't have it by default): https://testbot.winehq.org/JobDetails.pl?Key=143821
Actually, for that matter, if Media Center is just an advanced feature, is there any reason not to always return 1 here?