After dominating the desktop for a lifetime, the two-party state of Windows and OS X is under siege from a host of platforms. It's a process that continues to evolve as the definition of a PC broadens to a point where - legalities permitting - Samsung will soon have personal computing devices ranging from 5in to 32in and beyond.
With Windows 8 on the horizon, and made for mobile, things are set for another great leap forward. The question is, which horse should you back? History tells us that multiple operating systems cannot thrive in the same era. (And no-one wants to commit to one platform, only to take a bath when it fails. I'm looking at you, HP TouchPad owners.)
The safe bet remains Windows. After all, Microsoft's OS is king of desktop and laptop PCs, with around 93 percent of the global web-connected PC market. In the affluent UK, where Apple takes a larger bite of the market, the figure is a still whopping 89 percent.
But the world is moving on fast. Mobile is the future, as people in first-world countries increasingly demand a connected device at any time and wherever they are. Meanwhile those in the developing world use what we still quaintly refer to as 'phones' to get online. Where the infrastructure and personal wealth to hook up a desktop PC is lacking, a phone suffices. The mobile versions of Windows - although increasingly impressive - have never been totally successful.
Right now the various flavours of Windows Mobile account for only around a third of 1 percent of connected phones and tablets globally, with a similar number in the UK. As the trend toward mobile deepens, Microsoft is going to have to pull several expensive rabbits out of hats to head off a damaging decline. That this thought has occured to the Redmond mafia is obvious: why else would Microsoft make Windows 8 mobile orienated, and partner with Nokia (still the world's largest phone handset manufacturer)?
Viewed through a similar prism, Microsoft's long-term antagonist Apple is in clover. No fewer than 73 percent of mobile devices that connect to the web in the UK are iPhones or iPads. This number drops off signicantly when you include the developing world, but the house that Steve built is infinitely more interested in milking profits from the privileged few than it is in winning global popularity contests.
See also: Apple vs Google: and you win
That UK figure is so big, Apple's main problem is that it may be about to butt its head against the ceiling. As other platforms develop to match iOS' sheen and shine, and more people take their surfing mobile, those who chafe against the expense and restriction of being an Apple person may look elsewhere for a device.
I recently hosted an event where we introduced a group of people to the Samsung Galaxy Tab 10.1. All of them, without exception, were pleasantly surprised at the quality of the device itself, and the experience of using its Google Android operating system. The second generation of Android tablets is unrecognisably better than the first, and in the Samsung Tab and the Sony Tablet S the iPad finally has a pair of rivals worthy of the name. And while Apple may not wish to be the mass-market leader, Android's success matters. Why? In the post-PC world, the key to popularity (and money) is apps.
The end-to-end, curated world of iOS and the App Store is unsurpassed in terms of quality of user experience. If you download an app to an Apple device, it has been checked and approved by Apple. And you'll never find a malicious app in the App Store. But this locked-down, sanitised software eco-system is as much a weakness as a strength. It's hard to get an App approved by Apple, and even harder to turn a profit from it. Anyone can write an app for Android, and the signs are that software developers are increasingly training their guns on Google's OS. It's based on Linux, after all, and I'd wager that pretty soon you'll find more big-name software products optimising for Android over iOS.
For the user the choice is clear: Android offers flexibility, customisation and a burgeoning world of software, as well as malware and the occasional bit of interface clunk. iOS is beautifully crafted, a joy to use, and utterly restricted. To put it another way, Android is the new Windows, and Apple remains Apple. Where this leaves BlackBerry is anyone's guess, but RIM reminds me of nothing so much as pre-Steve Jobs return Apple: lots of products, lots of users, but no innovation and an uncertain future.
In fact, Google's now-found penchant for evil makes it a more than decent ringer for Microsoft. And for me, Android is where it's at right now. But I look forward to Windows 8 more than any Microsoft OS I can remember (and I can remember a few).
The challenge for Windows 8 is to keep up sales of PCs and laptops, and at the same time become a viable mobile alternative to Android. The reality is that if this fails, and the PC continues to recede from major player to digital hub, Microsoft may find that Windows is the new Linux: part of every computer setup, but used as a primary computing device by only a few.
See also: The 'post-PC world'. More PCs please