For all intents and purposes, in the gaming scene "PC" is but a synonym of "Windows" (with just two letters), but term PC encompasses x86(_64) Linux, Mac and Windows "PCs"... and even in that context it is used "wrong", PC is simply "Personal Computer" regardless of the architecture (PowerPC, SPARC, x86[_64], IA64, PA-RISC, MIPS, etc), so long as it remains personal.
At any rate, wasn't there a Microsoft "initiative" to label and market "Games for Windows" to give Vista big push?



according to this 
I actually make it a point to buy games that look half-decent and which support Linux, even if I almost never play them...)