Microsoft has delivered two customer-visible innovations to Windows PC's in the past decade:
1. Fisher-Price^h^h^h^hAero UI
2. Non-atrocious security and stability
The eye candy has never been enough to move units on its own. It looked pretty, but no one in their right mind jumped on the bandwagon for the translucent task bar. Microsoft did (mostly) fix the gaping security vulnerabilities though; A decade ago Win Rot was a given, just a matter of time. Sure, we geeks took that as the cue to wipe the hard drive and start over, but "normal people" bought a new PC, and threw the old one in the nearest dumpster or out of the way closet. Today, it takes a modicum of active effort (or ignorance) to spoil a Windows installation. People have to wait for the thing to actually die, hardware-wise, before they have an excuse to go shopping for a new one.
In the meantime they're snapping up tablets and smartphones, and are finding themselves on the computer that much less. I don't think too many people are actively ditching their PC's in favor of their iPad, but an awful lot of tablet owners are skipping the second (or third) PC for the house, and they're not urgently buying replacements when their PC's die, even if it was the only (or last one).