It's a bit odd really. I used to have three on my computer. One was sam, another was michelle, and I can't remember the third. I've reinstalled windows though, and they aren't there. Just sam. They might come with office. There is definately one called michelle, i'm just not sure where you get it. Sorry! I'll repost if I find out.
I have three voice options in my Text To Speech translator (as default) in WindowsXP Professional SP2.
There's Microsoft Sam and LH Michael and Michelle. Not sure where you could get more, though I imagine that if you purchased professional TTS and T-reader software you'd get a broader choice.