Here's an interesting article on TCO as it relates to deploying Linux or Windows. It's not the usual open source jihad, or the pro-MS kind of pap some other analysts seem to reflexively put out either:
"If you've got somebody who's smart and can config it, then [Linux is] a beautiful desktop and runs well," says Meta Group analyst Thomas Murphy. "But for the average business owner, [it] does not have that kind of simple nature that you have in Windows."
Past all the hoopla, that pretty much captures it. Linux makes a great server platform - but as a client, you need to have a knowledgeable person set it up. Yeah, I can hear the yelling already about command line driven vs. dialog box, and about how great Gnome is. Doesn't matter - For the average PC user - who's not (and doesn't want to be) a power user, Windows is going to provide a better and simpler "out of the box experience". Of course, if my Dad and the rest of the Mac faithful get their two cents in, these same users would likely be happier with a Mac. After watching my wife fight with the digicam software, I might be inclined to agree.