No offense intended, but I'm not sure what business of yours it is which computer your Aunt uses. Unless she's managing a trust fund for you, it's her money, and if she likes Windows, that's what she'll buy. If it's your money she's spending, get a lawyer.
Don't buy into the "switch" hype about "Mac's just work" and Windows "blue-screens" all the time.
Computer support has been my business for many years... PCs, Macs, Vaxs, Suns, SGIs. PCs rule when it comes not only to power for the dollar, but in some cases, for power period. If time is money in your business (including visual arts as well as engineering), the choice is PC. If serious gaming is the issue, the choice is PC. If budget is the issue, the choice is PC. If having a certain software, such as AutoCAD, is the issue, the choice is mostly PC.
Over the years, Windows has become increasingly more stable, and from my experience, when properly set up, XP is no less reliable than any other platform. The real issue is that there's crappy hardware out there, good hardware that won't coexist with other good hardware, and crappy software. If you buy from a major PC maker who sells only combinations of hardware they've tested, you're less likely to have problems... until you start adding software. That's no less true for the Mac.
Apple does have the advantage of only having to support a very limited number of hardware combinations. Despite that, there are still many problems. Sometimes they're because the user has gone beyond Apple's software, sometimes it's because they haven't.
If you want to show her how "Macs just work," let her wade through the literally thousands of Mac problems at
Apple's Support Discussion Forums. That's the reality any computer support person will show you.
The way the Mac does things suits some people better than others. Even from the beginning, some couldn't get over the idea of dragging a disk icon to the trash to eject it. And some people are more comfortable with Windows, or put more importance on software availability. Why should your Aunt have to relearn how to compute if she's comfortable with what she has? Macs do a LOT of things differently!
I'm probably going to switch back to Mac at home for the second time this summer. Part of that is because I like Unix, but don't want to HAVE to use it when I don't want to. Sure, I could dual-boot Linux and XP, but I also want to get away from Microsoft in my private life for personal reasons.
If someone asks, I'll advise. Sometimes it's Mac, but often it's PC. But it's none of my business what others spend their money on, just as its no one else's what I spend mine on.