As Hutch wrote, a C program would run on other cpus, too. But that is not what you are after: you want a program that runs on a Windows computer, period. Masm32 programs will run on any Windows computer (ok, show me one that doesn't have an x86 or x64 cpu...).
You have the choice, though, between:
- the Masm32 SDK (copyright Hutch, it is his baby): mature, runs on both x86 and x64 cpus, and practically all Windows versions
- one of the Masm64 SDKs (the most advanced one is from Hutch again, but that is a long story): in constant development, runs on Windows 64-bit versions only.
For a beginner, Masm32 is much easier, and it offers more functions. Perhaps you should explain a little bit more why you want to write Windows programs: for your own hobbyist use, or in the hope of selling something later, ...?