It's probably more accurate to say they've lost mindshare. The amount of PC marketshare they've lost to Apple is still pretty small.

But the biggest problem for Microsoft is that the market is shifting. Between smartphones and tablets, people just don't need PCs as much. So Microsoft is desperately trying to be a factor in those markets. Shrinking the PC down to the size of the Surface isn't a bad idea, but it will make OEMs very uncomfortable. They are feeling the same market pressure to get small and mobile.