Major problem there is the same as the mobile market - you have to write very fundamentally different code for a mobile and desktop. Some fascinating figures came out of this year's Siggraph. On the desktop you typically have up to 300W of power dedicated to graphics hardware. On a mobile device you have at most 1W (phone) or 5W (tablet). Those numbers will _never_ go up because anything more than that starts to fry your pocket or hand. So, the optimisation techniques that one uses to write a desktop app or game are extremely different to those written for a mobile device. There's just no getting around that at all.
One of the other interesting factors is that from a developer and graphics perspective now, except for the desktop gaming market, D3D is all but gone. I saw one mention of D3D at Siggraph this year, and that was because the chair of the panel was from MS. All those tablet/phone game writers were way over in the OpenGL ES camp. MS is trying to force the issue again like it's 1999 and now allowing OpenGL drivers on Win8, so you'll see how quickly the game studios will react to that - Even Valve were demoing OpenGL games this year that had better performance on Linux than on Windows on the same hardware according to their statements at one talk. Apart from business desktops, I don't see much more future market for the Microsoft and PCs. The games of interest are now appearing on mobiles and those developers are definitely not in the MS camp.
Summed up, it's a failed strategy and will do more to make people move away from MS than towards it.