There were two things wrong with Vista: 1) issues with 3rd party software & drivers, and 2) steep hardware requirements. The first was due to enhancements to user accounts & security, and is largely fixed today.
Anyway, people aren't going to be using Windows XP for the next 5 years, so they'll switch en masse to Windows 7 or to OS X. Apple still isn't competing at the lower price points, so unless Windows 7 is just really bad, I think we'll see the numbers pick back up when Win7 comes out.
If this trend continues to accelerate, the revenue Microsoft gets per PC will steadily decline and likely offset the increase in revenue from increased volume.
And Paul predicted this a while back.
Desktops would be replaced with a smaller web-focused devices. [1] Grandmas will be left using Windows. [2] Microsoft is declining in importance. [1] [2] [3]
[1]http://www.paulgraham.com/road.html