I've always run Windows, because my family does and until now I haven't thoroughly understood the choices to be taken when purchasing a PC - to be truthful, I've never had my own PC. I've used my dad's, and when he got a new one I got a 5 year old one for a small amount of time, but never really treated as mine. I got a laptop running XP for christmas, my parent's choice entirely. I've since swapped to another laptop, my parent's choice again and bought for my brothers initially.
Effectively, the PC I'm about to buy is my first real choice in purchase, and was exactly the one I asked for. I suppose it's fair to say I was influenced by my past use of PCs, and Windows was naturally my first choice. I did look into Linux and very quickly decided it wasn't for me without heavy customisation (I'm not a very experienced coder, and have never touched an OS). I did look into OS X, but for various reasons turned it down.
The first was that I had a couple of chats with someone I know who develops business software for cross-platform and also develops the Operating Systems for business software. (Similar to the barcode, and silly advertising you see on screens at checkouts sometimes). He said I should either buy custom or build custom, to avoid silly cheap pieces that the big companies put in to cut the cost. As he said, there's a reason they only gave the spec on that Graphics Card emphasised in big red letters, and the Processor's quad-core support, but not the Hard Drive's cache, quality of the Network card or PCI-E slots available on the Motherboard.
After not too long, we decided that custom was the way to go. He gave me a really handy collection of stats to look for, and explained that buying custom I can easily get any spec out of them, and building each piece should come with it.
Naturally, the stuck up people in suits at Apple decided it was a bad idea to allow customisation of their products by the end user, and that was pretty much the end of the possibilities of Apple. It's not available stand alone, and who knows what sort of crazy compatibility I'd get trying to change things in it? No thanks, I'll go with Windows.
Around this time, talk was everywhere about Windows 7 Betas. After a lot of research and discovery of some incredible features, Windows 7 was pre-ordered about a week before release. Been with me ever since. Always been PC, and I think I always will be Windows based PC.
While apple may offer the ultimate compatibility for their own pieces, it's about as good as buying a laptop. The possible upgrades seemed few and far between, and seemed to be a few hundred pound more than same spec PC. I'll go for future-proof and compatibility if you're careful, over self-compatibility only and dead-end.