People often speak of the "Wintel" duopoly, which of course is a misnomer. A duopoly is when two companies share a single market, at which point there is a risk of anti-competitive behaviour. However Microsoft and Intel are not a duopoly, they are two near-monopolies. Microsoft dominates the desktop operating system market, and Intel dominates the desktop CPU market. Although both OS and CPU are necessary components in a desktop computer, this doesn't make their manufacturers a duopoly because even in the best of worlds they don't compete: increased market share for Microsoft is not at the expense of Intel.
However this doesn't mean that everything can be all cozy between them, because in a broader sense they do compete. Microsoft and Intel are players in the desktop PC "value chain" (actually, its a value network, or even more precisely, a value directed acyclic graph, and if you look closely you find it isn't even really acyclic because chip-makers buy PCs, but term "chain" is more often used so that is what I'll stick to). This describes the way that money flows from PC buyers through vendors to manufacturers to parts makers to raw materials.
If you are a player in a value chain then its pretty much a zero sum game; every time someone buys a PC their money bubbles back up through the value chain and everyone grabs their bit. Your problem as a player is to get as much of this money as you can, which inevitably means that someone else gets less. Value chains are characterised by dysfunctional relationships between people who need each other but nevertheless hate each others guts.
There are two big strategic goals in a value chain. The first is obvious, the second less so:
- Make your bit a monopoly, so the rest of the value chain has to come to you to produce the product. That way you can charge monopoly prices and hoover up all the money coming through the value chain.
- Make everyone else's bits into generic commodities so that they can only compete by keeping prices low. That way they can't charge monopoly prices, which leaves more money for you. That's the consequence of the zero sum game thing.
- Microsoft makes very sure that Windows runs well on as wide a variety of hardware as possible in order to keep PC hardware a commodity business.
- Intel makes sure that Linux runs nicely on Intel processors in order to create a competitor for Microsoft, so Microsoft will have to reduce its prices, thereby leaving more money for Intel. I suspect Intel were also very supportive of Apple's move to Intel hardware for this reason, as well as the more obvious one of having a new channel.
- Microsoft made sure that Windows runs well on AMD. However AMD have never really recovered from Intel's "Intel Inside" marketing coup (where they paid box vendors to make it sound like "Intel Inside" was a big selling point, so lots of customers thought it was). So now Microsoft need to create a new competitor for Intel, and have decided that ARM will fulfil this role nicely. In the past they also did this with the DEC Alpha for the same reason.
Edit: "zero sum gain" -> "zero sum game". That's what happens when you let your fingers do the thinking.