Wouldn't this be all about raising awareness for Microsoft?
Sure, the majority of people use a Microsoft product, but the majority of people use this Microsoft product because they just think that that's what a computer is. I'm thinking about my parents here. They don't have a clue about computers and, just like in the adverts, they think it is a case of "PC" or Mac. Not Windows or Linux or Chrome or OS X... They're computer is a Dell or whatever. There's no mention of Microsoft.
If Microsoft actually makes the general non-computer-savvy public know that they make this OS, as a separate entity from the actual computers they (the public) are buying, then Microsoft can make them aware of its features, of its productivity uses, of their software, of all of these things' links to their other products... Zune, Xbox, their mobile phone OS... In this way, they are very much a similar company to Apple.
What i imagine Microsoft's aim is, is to make people aware that they actually make an OS. That Windows isn't just something thrown into, and made as part of, every computer that's churned out. That this OS has features and uses.
Going to their ad campaigns, "I'm a PC and i do this..." etc. That was a stupid mistake for them to make - but perhaps one they had to since their only other choices were to remain quiet or talk about an OS, Vista, that near everyone had written off. Even the shopper ads are silly, focusing on the hardware aspects they (Microsoft) have nothing to do with - again, this was really the only choice they had left at the time.
... (snip snip)... .