old argument
Argument goes like this: "Microsoft must make quality software because they are the most popular". Alternatively "Microsoft must be doing something right because they are the largest".
For the first one, I simply need to point to MacDonalds to show that just because something is "popular" doesn't mean it has "quality". You can sell crap to idiots and still be "popular", and companies like Microsoft and MacDonalds do just that.
For the second, all you have to look is Microsoft's history. They didn't get big because they were good. They got big because they were mean and unethical. Through a few shrewd (read: unethical and possibly illegal, such as selling something they didn't own) manoeuvres they got a fast start in the early 1980's. Since then they have bullied their way to the top, and repeatedly, maliciously and illegally used their effective monopoly in one domain to push their crap into other domains. Meanwhile, their government let them because that kind of behaviour is considered "good" and "successful" in that culture. The bully is finally starting to see some of the rightful penalties for that kind of behaviour in countries where the rule of law means something, but it's been very slow in coming.
As for XP vs Vista - you're right, there isn't much difference. One is just as broken as the other. However, at least XP has the background of people and software to make that broken piece of crap do vaguely what is required. Why should people pay more money to replace something that works for them with something that provably doesn't work? Why should people spend just as much switching from one lockin to another lockin as they would from switching to a system that would not lock them in so thoroughly, and would give them the option to move at their own schedule in the future? Personally, anyone working for me who failed that particular basic accounting (not to mention ethical) test would be walking the plank. Luckily, I've only hired intelligent people so far and haven't had to do that.