I don't think there are that many viable outliers; this sounds like the "if just 1% of X people, and just 1% of those ..." argument that some young businesses use as their market research.
Let's consider some of the demographics and logic here. First, we're specifically talking about XP users. Vista has been available for quite a while, and despite all the bad press and the vehement hatred that the tech sector has for it -- a lot of users aren't all that bothered by it. It works OK.
So, if someone is still using XP, then it's probably safe to conclude that either they're apprehensive of change, or that they don't consider their computer to be something that they should spend much money on.
In both cases, Apple loses at converting them. Macs are more expensive, and the operating system is completely different from the user's perspective.
The only outliers that leaves us with is those folks that have more than one computer in their home, of which one or more of their systems is XP. In those cases, either they already have a Mac in their home -- and Apple isn't winning a conversion -- or they don't, and Apple is not really likely to get a conversion.
But all that's just theory and guessing. Fortunately, one of my two businesses currently is in tech services for individuals and businesses. Of the roughly probably 100 systems that we currently care for, just 9 of them are Macs. Of those, 7 are used by people who will give up their Macs when you pry them from their cold, dead fingers. (Which is fine by us, we like -- and support -- Macs too.) Of the remaining two, one is an older laptop with persistent hard drive and charging issues which almost drove the client to get a PC replacement.
The other one is a more interesting story related to what we're talking about here. The client was persuaded to get an Apple at the behest of a friend who really really likes Macs, and is one of those misguided evangelists that believes that everyone else should like them too.
The problem is, this client doesn't play with computers. He just wants to get work done. So, one of the first things he did with his Mac was get an early beta of Boot Camp -- and install Windows on it.
Apple since then has done this really neat trick where a bug in that version of Boot Camp prevents the user from upgraded to XP service pack 3, and you can't upgrade Boot Camp to fix the bug without buying Leopard.
Meanwhile, the user is frustrated as all hell because the machine is really quirky. He hates it. He just wants to get work done, not dick around with it all the time. To him, the Mac is always slower than his other PCs, it's always more troublesome, and it was more expensive.
As a result of that "conversion", Apple has just won an enemy for life. He'll never buy one again.
I used to do the whole "you're better off with a Mac" evangelism thing -- essentially doing Apple's marketing for them -- years ago. That got old pretty fast when all those people started coming back to me with complaints, and most of their complaints distilled down to, "it's not Windows!"
Now we recommend people just use what they're familiar with. They're much happier that way.
The marketing strategies from both Microsoft and Apple right now are completely irrelevant to their prospective customers.