Apparently Cortana has passed early testing on Windows 9 (aka Windows Threshold) for PC and current plans are for the voice controlled assistant to be included in the update.
The rumour says that the Cortana app on PC takes up about a quarter of your screen, so you’re still free to do other stuff. And it’ll accept written and spoken commands and questions, which could make your PC work hands free.
It’s possible that a preview of Windows 9 with Cortana will be ready by the end of the year. If it is, it’ll be exciting to see how it helps PCs and Windows Phone mobiles work better together.