Cortana has been in Windows Mobile for a while -- not sure it's actually all that useful on the desktop. Uptake of talking AI has been generally poor, even on phones. I can't remember the last time I saw someone talking to Siri, though I've seen plenty of people swiping their phones.

http://www.ibtimes.com/apple-ios-7-85-percent-people-havent-used-siri-46-percent-think-apple-oversold-its-release-1437900