How To Disable Cortana in Windows 10
1. Open regedit the registry editor, from the search box on the taskbar.
If asked, allow the program to make changes to your computer.
2. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Windows Search
But wait! Windows Search might not be there. It wasn't for us, so we had to create it.
2a. Right-click the Windows folder and choose New>Key. Call it "Windows Search."
3. Right click "Windows Search" and choose New > DWORD (32-bit Value).
4. Name the DWORD "AllowCortana." Click it and make sure the value is "0."
5. Restart the computer (or log out and log back in). Cortana will be replaced with a regular search bar.
While Cortana is gone, it's still hiding. You may find the program running in the background, but you might as well leave it there. When we tried killing the program, Cortana kept popping back up like a weed. When PCWorld managed to kill the process, they couldn't search for anything on their PC.
It's possible that the next Windows update will restore Cortana to its place in Windows 10, so be sure to keep these instructions handy, just in case.