I'm guessing you're very young. NT used to be available on x86, MIPS, AXP and PowerPC. It was developed not on x86 but on i960, a RISC processor from Intel. There were plans for a SPARC version too but endianness issues in the HAL meant it was never performant.
The only reason Windows only runs on x86/x64 now is that customers weren't interested in it on other platforms. Microsoft really tried to make it cross platform.