In addition, third party developers have always enjoyed free, open access to the same set of system APIs as Microsoft itself, so it has been an easy platform to develop for. That, in turn, has strengthened the positive network externality that keeps Windows dominant.
The reason it is impossible to buy a computer with Linux is largely Mircorosfts wildly anti-competitive behavior in the past. Note how much work and marketing it took Apple to carve out a non-trivial market share--there are no companies pushing Linux adoption nearly as heavily.
There are too many confounding factors affecting Linux on the desktop to use it to judge open technology in general.
Overall, Linux has lower OS market share than five other operating systems, all of which are either from Microsoft or Apple. http://en.wikipedia.org/wiki/Usage_share_of_operating_system...
I'd like to believe that consumers will, in the end, choose open technology (directly or indirectly). Maybe it has something to do with the rate of technological change, and it's just a matter of time before the benefits of open technology are demanded. But the evidence is not conclusive.
If anything tips the scales, it will be when consumers start to figure out that they don't own their devices or the content on them. People are getting a vague sense of that with locked-down mp3 players that make it hard to copy songs around. It will be a bigger deal when the person that gives all their books away because now they have electronic ones becomes furious when they find out they are forever beholden to amazon (and its book-deleting whims) to be able to read them.
There are other factors, too. I'm just saying that it's not entirely one-sided.