Compile 'em all said:
Bottom-posting is the defacto standard on most usenet groups and technical mailing lists. YMMV of course.
Yeah, for some reason, this dichotomy has grown as the systems for e-mail and forums / newsgroups diverged. Because (I know what you said about mentioning Windows) on desktops of a variety of platforms, people use top-posting, and I'm pretty sure most web-based e-mail clients also top post. It used to annoy me, and I used to manually shift things around, and then after a while, I said meh, and started going with the flow. On the web, I bottom post. In e-mail, I top post.
However, unless you want something different than I think you want, I think you can actually get it from Mail.app. Go to the preferences, and then signatures. At the bottom, uncheck the box "Place signature above quoted text." When you reply to a message, you now get the quoted text first and the signature last, so that you can insert your comments in. Is that what you want?
PS...welcome to the fold, even if you're only partway in!