81% of lost Windows market share went to Mac OS X
The following graphics tells the story:

The gain is almost directly proportional to a declining Microsoft Windows install base.  It's evident that Windows' loss is OS X's gain.

Switcher Effect
When all other operating system's with at least 0.01% market share are analyzed, it's clear that Mac OS X is experiencing growth at the expense of Microst Windows.  In fact, two of the top three gains on a percentage basis were for Apple; Mac OS X and iPhone.  

The Switch To A Mac phenomenon is continuing to gain momentum.  Since the inception of this site in January 2006, we have put forth the opinion that Mac OS X was to experience significant market share gains.  The most recent data from Net Applications validates those views.  We have listed a few related stories with our published views on market share and Mac growth below.  So it begs the question, does the data hint that of those dropping Windows, do 4 out 5 leave Windows for Mac?

Mac OS X grows 26 percent year-over-year
In November 2006, Mac OS X had a 5.39% share of the operating system market.  The November 2007 data shows that Mac OS X market share grew 26.15% year-over-year.

Related Stories

About Market Share
Net Applications, in contrast to firms such as Gartner who uses quarterly industry sales results, determines market share values based on a network of Internet users.  You can read how Net Applications obtains thier data here.