I have read one insider account that Intel had its own, different x86-64 instruction set, designed in response to AMD's. It approached Microsoft and asked it to port Windows to it.

Microsoft refused, saying "we already support one failing 64-bit architecture of yours, at great expense and no profit. We're not doing two just for you. There now is a standard x86-64 ISA and it's AMD64, so suck it up and adopt the AMD ISA -- it's good and we already have it working."

Or words to that effect. :-)

I've not been able to find the link again since, but allegedly, yes, the success of AMD's x86-64 has been due to Microsoft backing it. It sounds plausible to me.