On Android 6 you can change the app for Assist and voice input to Cortana for ex...

ewzimm · on April 29, 2016

In the exact same way, you can change the app you use for searches on Windows to Google Now, even disable Cortana completely.

zacklytle · on April 29, 2016

Actually the latest insider build (14332) removes all options to disable Cortana. Before you had to opt-in with "get to know me", now it's on at first boot and can't be turned off.

seanp2k2 · on April 29, 2016

It's no longer possible to rid Windows 10 of Cortana without breaking the start menu and search bar: http://superuser.com/questions/949569/can-i-completely-disab...

I recently tried to remove Cortana, because I personally find voice search worse than useless. I don't want my start menu searches being sent to MSFT. SteamOS for gaming can't come soon enough.