Apple and Microsoft have a rivalry that is the stuff of legend. In recent years, though, each company has come to dominate separate arenas. Microsoft still owns the desktop PC market, while Apple has the most popular smartphone and is the undisputed ruler of the tablet.So the Apple/Microsoft rivalry has fizzled a bit. Apple even stopped running its Get a Mac (“I’m a Mac,” “I’m a PC”) ad campaign a couple years ago. Each company now has its bread and butter, and those niches aren’t necessarily in direct conflict with each other.But that is about to change. Both companies envision the desktop and mobile spaces converging. Naturally, their views of how this will happen coincide with their respective strengths. Microsoft, the desktop market leader, wants Windows to seep into mobile. Apple, makers of the iconic iPhone and iPad, naturally wants its mobile ecosystem to bleed into the desktop.So the stage is set for the next great Apple/Microsoft showdown. This year we will see the increasingly similar Windows 8 and Windows Phone do battle with the converging OS X Mountain Lion and iOS. It will be a battle of opposing halo strategies.Round oneMicrosoft has already previewed its first line of attack: Windows 8. The operating system will run on both desktop PCs and tablets. The tablet UI, in turn, will look nearly identical to its smartphones. It’s a bold strategy that begins the unification process and will signify the biggest fundamental shift for Windows since Windows 95.OS X 10.8 (Mountain Lion), meanwhile, will continue the transformation that began with last year’s Lion. iOS features like Notifications, AirPlay, Messages, Reminders, and Game Center will all show up in OS X. iCloud will be the glue that holds it all together. The cloud service will let Apple retailers pitch long-time Windows users on the ease of having their iPhones and iPads sync effortlessly with this shiny new iMac or MacBook Air.Opposing strengthsOn the desktop end, Microsoft has a huge advantage: price. No matter how popular Macs become, they are still much more expensive than your average Windows computer. Until Macs are routinely sold for $500 or less (not likely), Microsoft is going to retain a large share of the desktop market. Many customers don’t want anything fancy, they just want something that gets the job done and doesn’t break the bank. That is most often a Windows computer.Apple, on the other hand, has customers’ imaginations on its side. Apple product announcements have become pop culture landmarks. The company’s iconic products have bred a collective passion for its devices that Microsoft can’t come close to rivaling. The iPhone and iPad have grown to exemplify the 21st century merging of technology and lifestyle. Apple wants to take that momentum, and carry it into other areas — including Macs.The gateway drugApple has another distinct advantage: the desktop and the smartphone are at the extremes of this “one OS to rule them all” shift, but more crucial is the middle ground. It will be the gateway drug that leads customers from one end to the other. This middle ground is, at present moment, utterly dominated by Apple. The middle ground is the iPad.Microsoft knows this, and is scrambling to turn the tables before it’s too late. It’s hoping that Windows 8 will play the Pied Piper, ushering legions of desktop users towards Windows tablets and smartphones.But Redmond will have its hands full. The only tablet that has gained any traction — that isn’t an iPad — is the Kindle Fire (which doesn’t necessarily compete with it directly). Amazon accomplished that by undercutting the iPad at a lower price point. Windows 8 tablets, however, won’t be $200 color readers. Instead, they’ll take the same route that countless Android tablets have: going head-to-head with the iPad on the high end. None of those tablets have gained any significant market share.Microsoft’s ideal futureIn Microsoft’s ideal future, consumers will eat up Windows 8 on the desktop, and fall in love with the Metro UI of Windows 8 tablets. Even if the iPad is still the top-selling tablet, the goal is for Windows 8 tablets to take away from that invincible iPad mystique, and give customers the perception that there are lots of worthy tablets. If Microsoft can do that, then it will have some solid footing and can use the volume and variety that come from a licensed platform to eventually lead the market.This will be done alongside a heavy Windows Phone marketing push. Though Windows 8 and Windows Phone are still separate platforms, the Metro will unite them in customers’ eyes. The success that Windows finds in tablets (the gateway drug) will naturally lead customers towards Windows Phones.Microsoft will perform an epic comeback, using its stronghold in the desktop market to make up for lost ground in mobile.Apple’s ideal futureIn Apple’s ideal future, everything hinges on the iPad continuing to dominate. As the tablet market eventually outgrows the PC market, Apple will lead the way. This is an end in itself, but the company will also build on its share of the shrinking PC market. Customers will dismiss Ultrabooks as pretenders, and the iPad will serve as a halo device that leads former Windows users towards, first, the MacBook Air, then the MacBook Pro and the iMac.In this future, Microsoft will continue to fall behind. It will find its niche, but its days of domination are over. The PC market will shrink, and Windows’ share of it will dissipate as well. Its tablet and smartphone attempts will be too little, too late. RIP to the Gates-era Microsoft of old.Will we have a winner?Of course this won’t be a winner-take-all battle. Even if Microsoft’s tablets and smartphones flop, Windows isn’t likely to lose its leading position within the next five years. Likewise, even if Microsoft’s desktop halo strategy does give its mobile devices a boost, Apple isn’t going anywhere.This battle isn’t going to be decided overnight. Two years from now, the desktop, tablet, and smartphone markets probably won’t look much different. Ten years from now, though, we could see a changed landscape — possibly even a radically changed one.No matter how this saga plays out, it’s guaranteed to rekindle a classic rivalry, and will be loads of fun to watch. Throw Google into the mix, and we have a geek’s steel cage match for the ages.