Market Share vs Installed Base: iPod vs Zune, Mac vs PC: Daniel Eran takes aim at claims of market share. He shows that the only thing the numbers prove is that one can create statistics to support any hypothesis, given the opportunity to tweak the criteria.
More Macs Mean Many Fewer PCs

The worst news for Microsoft is that every new Mac sold to a former PC user means that at least two Windows PCs will go unsold: the initial PC and its mid-life crisis, second string replacement.

Compared to the 240 million PCs sold last year, Apple’s small share of around 6 million Macs per year won’t put Microsoft out of business any time soon. However, it means that 12 million Windows licenses will go unsold. That’s up from less than 4 million Macs eating up 8 million potential Windows licenses just two years ago.

The faster Apple grows, the more devastating this effect will be to Microsoft's automatic sales machine, which in turn will help open the market up to new choice and options, including Linux desktops. Once Microsoft loses its monopoly control of the OEM market, Windows will fare as well in the market as the Zune.

A prime segment of choosy consumers are going out of their way to not buy Windows. Microsoft doesn’t want to dominate the low end of cheap PCs; that market is already headed towards Linux anyway.

Microsoft wants to retain the valuable top tier of the PC market, the segment rapidly buying new Macs.
This is more news showing that, despite the bragging, Microsoft's Windows Vista is unwanted by consumers and corporations.  Apple's unit sales rose significantly over the past year, and each Mac tends to last about twice as long as a PC.