That is what came installed on the computer is a major factor also. It may be the most important one in the home market and is a significant factor even in business markets.
When I'm asked for a computer recommendation, the first thing I ask is "what will you be using it for". OS is secondary. Trouble it, most of the popular apps are Windows based.
Where WE have an opinion about the OS, the average user, my wife is great example, just want to turn it on and run. OS/2, Windows, MAC, BeOS, GEOS, Linux, or DOS, they really don't care.