Unlike earlier Windows 10 versions, the new one (1809 at the time of this writing) doesn’t have any On/Off switch to enable or disable Cortana. In order to turn it off, you must modify Windows registry.

Here’s how:

WARNING: Modifying the registry incorrectly can make your Windows behave unexpectedly or permanently unstable. If you’re unsure what you are doing, or have little or no knowledge about how Windows registry works, you’re strongly advised NOT TO PROCEED. The author of this guide, AnandTech forum, employees, or any of its direct or indirect associates cannot be held responsible if something goes wrong to your Windows or computer hardware while incorrectly performing the tasks explained in this tutorial.

Step 1
Get to Registry Editor

Type REGEDIT in the Cortana search box, press Enter, and click Yes in the User Account Control confirmation box. This opens the Registry Editor window. All the configurations, settings, and other minute details of all the apps and Windows itself can be found here, and can be configured/modified to customize and personalize your Windows 10 that can best suit your preferences.

Step 2
Get to the Windows key

Type Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows in the address bar at the top, and press Enter. This selects and expands the Windows key in the left pane present at the location you specified. This key contains various other subkeys that participate in optimum performance of the OS to give you the best end-user experience.

Step 3
Create a new key

Right-click the Windows key from the left pane, go to New, click Key, and name the new key Windows Search. This creates a Windows Search key under the Windows key. This new key can be used to customize Windows 10’s search options and features.

Step 4
Disable Cortana

Right-click the Windows Search key, go to New, click DWORD (32-bit) Value, name the new DWORD as AllowCortana, press Enter, double-click the AllowCortana DWORD, ensure that its Value data field has 0 as its Hexadecimal value, and click OK. This disables Cortana on your Windows 10. Upon this, you must restart your PC to allow the changes to take effect. To re-enable Cortana, you can get back to Registry Editor (use the Magnifying Glass icon from the taskbar now available instead of the Cortana search box (or press Windows + R keys to launch the Run command box) to get to Registry Editor), and set the AllowCortana DWORD value to 1.