Android may actually have a chance of beating Windows
Granted, it is several years out, but I think android is the leading contender to supplant windows–even over Ubuntu and OS X.
One of the largest drivers keeping windows entrenched is that it is what people are used to. Many argue the superiority of OS X and Ubuntu and have for years, but for the majority of users, they are both are not-quite-right-clones of windows. Buttons are in the wrong spot, Office doesn’t work the same, some websites don’t render right. It doesn’t matter what the merits of the other operating systems are, they loose because they are different. That is the reason Windows-based netbooks are outselling Linux netbooks.
Ironically, however, that is the very reason that causes Android to pass windows. As phones become less of a phone and more of a computer on your pocket, the dominant phone OS will become the OS that users are used to. At that point, it will be Windows that will become not-quite-right-clone. At that point, the average user will choose an OS that makes their laptop work like their phone every time.
Why Android, and not iPhone OS? At WWDC, Apple talked about how OS X sales haved spiked since release of iPhone–which may corrabolate my hypothesis. However, I think that in the long run, the Apple controlled iPhone will not be able to compete with the more open, multi-vendor Android just like they could not compete with the more open Windows/PC. the iPhone will have a good couple years, but in the end they will be a niche player.
Why Android, and not Windows Mobile. Although Windows Mobile makes the same arguement I am making, but in reverse (look: your phone works like your laptop!), scaling applications and interfaces down is much more different and difficult than scaling them up. Mictosoft has had a long time to create a popular mobile OS, but they have yet to deliver because they are too connected to desktop paradigms to create a compelling mobile platform.
Not even Office will be able to keep users on Windows, like it does now. With users expecting to access their documents remotely and from a large variety of devices, office applcations are going to be forced to a cloud based/multi-device platform. It may be MS, Google, or someone else who delivers this, but whoever it is will need to have a great Android option available, which will stop Office from being the the Windows-lock-in product that it is today.
Will Android pass Windows as the dominant OS platform? It is hard to say, and if it does, it will be a long, long time. Netbook manufacturers are just experimenting with it, and even phone manufacturers are still experimenting with it. If anyone is able to pass Windows, however, I think that Android has the best chance of the choices available today.