Window's won't always have such a large market share. In fact, very few products have ever held such a large market share w/o government enforced monopolies. In a truely free market, such a monopoply is impossible.
MS is beginning to lose ground to Linux in the area of network servers and to Apple in desktops. Note that Mac OS/X is built on a FOSS operating system.
The vast majority of computers in the rapidly developing countries of China, India, Mexico and the entire continent of Africa, will wind up running FOSS simply because of the costs associated with proprietary software. China, India, Taiwan and Japan have already started talking about working on a Linux version specifically for their governmental needs.
The next generation of programmers from those countries are going to be trained first on Linux and other FOSS software.
So no, I don't think MS will be able to compete, not for the OS market.