Is it a PC or a personal computer?
In the old days PC was simply an acronym for personal computer. Then came along the IBM PC, and then the clones of the IBM PC. Overnight, PC became synonymous with IBM PC-compatible.
Now that even Mac from Apple is Intel-based, we may be on the verge of a landscape where the hardware distinction between PC and non-PC becomes moot. If I have a notebook computer sitting next to a Mac notebook computer that is running Windows, it seems rather silly and irrelevant to say that one of them is not a "PC."
The unbiquitous "I'm a PC. I'm a Mac." ads from Apple may finally institionalize the Mac vs. PC definition for PC, but I'm not so sure.
For the most part, I try to use the full term personal computer when referring to simply any box regardless of whether it is a "PC" or a "Mac", but I keep running across situations where I would like to simply refer to PC as a shorthand for a non-specific personal computer, such as a web client where the hardware and operating system are simply irrelevant.
Maybe the thing that really bothers me is that PC and Mac are simply not comparable from a categorical perspecitive. "Mac" is a brand name and a registered trademark. "PC" is simply a type of computer (compatible with the IBM PC architecture), a generic term, distinct from whatever brand a particular computer hapens to be.