Come now. Let's be realistic.
My mom is an above-average computer user because she does a lot of desktop publishing and internet-based work for her job (she's a real estate agent). Next time she needs a new computer, I tell her I can build one cheaper that will be better. I put Ubuntu or Fedora Core on it (these have been the easiest distros to use for me). Let's say all of the hardware works on the first try (this has NEVER been my experience, but I'm a hobbyist, I always have obscure hardware).
I leave my mom. She tries to do work. She can't because something's gone wrong. She calls me. I don't know what to tell her. She calls any other computer people she knows. Chances are, they don't know what to tell her. She could go to Best Buy and they wouldn't know what to tell her, and then they would tell her to buy and install Windows.
She could go to online forums, assuming she even knows how to phrase the questions she needs to ask or Google, but I'd say that she has about a 90% chance of finding people who are entirely unhelpful who want her to "help herself." She doesn't care about the FOSS movement, or how "technically superior" the Linux kernel is, or any of that. She just cares about doing what she needs to do for her job. Linux simply is not appropriate for it. To some extent, Mac is better, but it's still tough. Why? Because Apple has closed Mac OS X deployment to only Apple hardware, and there are significantly fewer people with real Mac experience to be able to assist.
"Free" software ends up costing my mom more in time, money, and aggravation because she is *forced* to learn the idioms. She just wants to learn enough to get by.
Until either everyone in the world has the time and wants to learn more than enough to get by, -OR- "free" software begins to get widespread, intuitive support (I guarantee you my mom would never click the "man pages" link on the menu, because she'd think it was an electronic version of Playgirl or something), then Windows is going to have an easy monopoly.