Apple has a head start. Android is building, and Msoft is coming in late next year. Sounds as though they are late to the party, and yes they are.
They missed the market with W7 cos they never built any touch into the heart of the OS. The problem is bigger than that, coz they have to carry so much baggage in the kernel and bolt on after bolt on to provide compatibility that the legacy of NT to W7 has made the OS in inefficient behemoth.
smart thinking would have extened W7 for mobiles and built out a replica of iOS / OSX model and seperate the ecosystems (gawd I hate that word)...
They will still insist that the can shoehorn W8 onto every platform under the sun with a confusing array of versions and licence models.
Now, this is still not so clear cut because as someone else has mentioned Business investment in Windows is here to stay for a very long time (business software, legacy apps, the sheer cost to move to anything else is prohibitive on a cost front - spreadsheets alone would make your mind explode at the number of them running core processes across the world today) no matter how much you think it is fixable it is not. In addition to this the synergy of an office PC and an on the go buisness tablet running the same will continue to be attractive for large organisations for support, security (hah) and legacy applications.
Now longer term there are very young people going through education, at home etc, that are learning their basic computing on non windows platforms - my 4 year old figured out the iPad in less than 2 minutes. Over a loooonnng time this will have an effect... so will cloud.
The transition will take years, companies will begin to develop their next versions more platform agnostic to ensure they can run on as many of these devices as possible.
Microsoft can afford to come to this late, and it will take a long time to see any shift. The consumer is irrelevent RIGHT NOW, corporations matter. Corporations are hooked in no matter how much they desire not to be. But today's four year old iPad / Android user is the possible CIO / CEO if the future, and this is where Microsoft is loosing them ... right here right now.