Maybe the absurdity of it all doesn't smack you in the face until you recast it in terms that have nothing to do with computers. Imagine hollering invectives at someone on an English-language message board because they have the nerve to read more books that were translated from another language than anything written in their native and beloved English. You'd think, "Why is this an issue? The more good books, the better, no matter where they're from." And you'd be right.
If you're passionate about something, it's all too easy to get myopic about it. One of my other big hobbies is movies -- not just any movies, but the best of the breed, the classics offered through the Criterion Collection and other labels. I have to accept that many people today have never seen or heard of films such as Citizen Kane, The Seven Samurai, La Dolce Vita, or M. I don't get angry about it. I do try to introduce people to the classics casually, as an adjunct to what it is they like now -- since they're often familiar with more recent films that are arguably just as good (Alien, The Shining, GoodFellas, and Blade Runner). If they get bitten by the bug, great. If not, well -- you're not always going to get the reaction you want, even if you are civil.
The same goes for the OS world. If someone's having a lot of PC trouble and doesn't have a lot of money to throw around, I introduce them to a variety of Linux as a possibility. If they pass and decide instead to drop $600 on a new Dell (with Windows), then I'll roll with that and show them what I know about how to get the most from Windows and not repeat their previous mistakes . . . and maybe hook them on some free/open-source software for Windows in the bargain. If they go Linux, then I show them how to make the best of it, too. Each one's an opportunity to get it right in a new way.
It doesn't get me riled up when someone chooses Linux over Windows or vice-versa. I've done enough of choosing both in my time to know that in each case someone typically has a legitimate reason for doing do. Because for most people it is simply not that big a deal. And it shouldn't be. The OS, and the computer, are just a means to an end for them.
I wrote before that Linux users should lead by example -- and, come to think of it, Windows users should do the same. Each one has something to demonstrate: that Linux can and should be an easy and viable alternative, and that Windows can run with great stability and security. Nobody is going to prove to the other side the merits of their way of doing things through insult, invective, or spluttering sophomoric insults.