Three years - at most. The game is up. People are tired of throwing software away and buying new one because MS decides it needs to keep their cash stream going. People see that they have to keep learning new systems every 2-3 years to do this and are tired of it - companies as much as individuals. Add to that the EULA, a self inflicted poison pill administered to try to keep the cash flow increasing and you have disaster for MS.
Linux is getting better and easier to use by leaps and bounds. You do not to have to relearn it either. It also comes with 90% of the stuff people use already installed and ready to go and you do not have to pay extra for it either. A good example of how well Linux is developing is Mozilla which took the code from Netscape and now has a much better product than the AOL backed product and IMHO a better product than IE. It may not load as fast but it sure has a lot fewer virus problems and lots more features.