PC isn't just a general term for "personal computer". It refers to an actual hardware architecture that's based on the old IBM PC architecture. Windows has nothing to do with it. Windows is simply an operating system. You don't have to install Windows on a PC. Besides, there really isn't any reason to specify "Windows-based PC" when Mac is practically unanimously referred to simply as a Mac.
The term "personal computer" itself is outdated. It was used when computers were first being marketed that regular people could afford and use practically. Now days, it's a given. If you're not buying a "personal computer", you're probably buying a server. There is no longer any reason to make the distinction that the computer you use at home is, indeed, a personal computer.
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