Platforms and Windows and Linux
Well, it's a bit more complicated than that... Porting stuff - applications rather than OS - is what I do for a living, and I've been watching processors go by for a while, and taking notes.
People forget nowadays that Windows NT started life on the Intel i860 (Huh? went the chorus), with MIPS as its second platform, and shipped as a four-platform CD for x86, DEC Alpha, MIPS and PowerPC. And all of those platforms except x86 died. The basic reason was that x86 became the fastest, and Windows' requirements grew fast enough that using anything except the fastest became silly. XP is Windows NT version 5.1; Vista is version 6.0. Windows 9x was throughly non-portable, but it's dead.
The world is now down to just two architectures that are performance-competitive and potentially mass-market: x86 and PowerPC. IBM weren't interested in doing PowerPC laptop chips, so Apple switched to x86. That's the vast mass of Linux systems, too. Itanium is on the way out: it never really took off.
So what platforms does Linux actually have a competitive advantage on? Embedded systems, micro-portables, and the like. The issue there isn't that Windows couldn't be ported to their CPU architectures, it's that it takes too much CPU power and memory, and thereby wattage, to be a cost-effective choice. Even Windows CE, which ran on many platforms quite recently, even if it is mostly ARM now, needs a chunkier system than a stripped-down Linux, or other embedded OS. That's the killer. Not portability, as such.