Fighting Windows users in nerd-to-nerd combat isn't easy, but it can be done.
First of all, you must assess your level of knowledge and your ability to recall that knowledge on command (something I'm not very good at). This can be a vital role in winning an argument with a Windoid. Remember, there are many variables to arguing about anything. You may know everything about Mac and Windows but may be an anti-social freak who can barely remember his name while talking to other people. If you are one of these people, the only way you can fight is this.
If not, keep reading.
After you have assessed your own knowledge and ability to argue, you must assess your opponent's. Keep in mind that they do not play by the same rules. They have the advantage of ignorance and sometimes (*gulp*) numbers. Also, they have probably been told their entire lives that Mac is inferior. If you do actually get through to them you will most likely be met with more hostility, the hostility borne of denial.
If you have had this friend for a while you have probably experienced the dislike they have for Mac and you've probably tried to avoid the subject because of it. Luckily, a friend is usually the easiest person to convince. You can start off with a look of a surprise and a comment like, "Oh, you're still using Windows?" as if Windows has become unpopular. Mixing a little amusement into this also works well.
If your friend has been totally brainwashed it will be harder, but it can be done. This may not be one argument, it may be many. It may take over a year (I have friends I'm still working on). An uneducated friend is like shooting fish in a barrel.
Listen to his argument first and then counter (because you have been educated by this site). If he said something like, "I don't like Macs because they're not customizable," say something like, "At the Apple Store you can practically build your own computer."
Continue to widdle away at his confidence in his OS in this manner (but don't be too aggressive!) and in most cases a good friend will eventually give out.
You may be chatting on the subway or this person or they might be in your computer class (a cumputer class where Macs are being used usually hurts your argument, since school Macs are usually not very well-kept). Wherever you meet them, this kind of argument can be deadly. They may complain about that fact that there are no games for Mac, or they may ask why you have to go to special, eject just to get a disk out (yes, I know, the oldest arguments in the book) but you will be able to easily counter. Many Windows users have sadly never heard of Mac OS X and still think Macs still run OS 7. This can be a great tool for you. If they have never heard of or used OS X that is a major advantage. This line works great: "You've never heard of OS X? How can you argue with me then?" If they are the classic Mac-hater their argument will probably degenerate into "Mac's suck!" and other such shitslinging.
At this point you have won the argument, and they either don't know it because they're too ignorant or do know it and a) won't let you show it or b) are still trying to convince themselves that you are wrong. Many Mac advocates make the mistake to keep arguing at this point. View it as a hit-and-run attack. You made your point, now you're done.
Direct them toward this site or another large source of information and get outta there before they call their geeky Windoid friends.
Windows users love to hang out in nerd herds and talk about the next joystick, the new graphics card, or the hot new Xbox game. Sometimes, although rarely for some reason, they will discuss the supposed suckiness of Macintosh.
If there are more than three of them it is smarter just to hold your tongue. Leave the room if you have to. But if you are feeling brave, quickly assess who is the leader of the group (this could be who started the discussion or perhaps the one that thinks he knows the most). Wait until he has said something that is untrue or as close to untrue as you can get and quickly debunk it. The three of them may become hostile or simply try to dismiss your argument. Throughout the argument, focus on that one person and use the tactics you've already learned. You can turn your attention to others momentarily if they try to make a good point, but keep your attention mainly on the leader. If you can't determine who the leader is, select what you consider to be the best anti-Mac arguments and snuff them out one by one.
This also greatly depends on the experience of the group. For instance, they may be Microsoft employees or they may be your average street geeks. Either way, don't panic and keep a cool head. Many people make the mistake of exploding under pressure. This will only hurt your argument.
A very good question. In the early days I used to design infocards and print them up to hand out to my friends (and enemies). Now these are available to everyone. As the argument is wrapping up or if you don't want to start in the first place, hand your opponent one of these. Don't be forcefull, only give it to them if they want it. Many will rip them up and throw them away, but it is the ones that will at least accept them that are potential ex-Windoids.
The same philosophy applies to arguing against Windows in general. Keep an even head, keep your facts straight, and keep in mind that these people are generally full of crap. Being a Linux or other open-source user can be a great advantage in this situation. You can still use Macintosh in your arguing. For instance, if your opponent says, "Windows is better because its so complex," you could say, "Mac users are 50% more accurate and 44% more productive than Wintel users. How much more complexity do you really need before something becomes overly-complicated and unuseable?"
This website has plenty of anti-Windows stuff, although most of it is also Mac-oriented. Check out the Anti-Windows/Microsoft Links Section for more general anti-Windows crap.