To disable Cortana and get the old search icon back, start regedit.exe and navigate to the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Windows Search
Create a new 32-bit DWORD value named AllowCortana and leave its data value as 0.
Since Windows 10 v1803 (Build 17134) (i.e. Windows 10 - 2018 Spring Update) you also need to add the DWORD BingSearchEnabled and CortanaConsent values with a data value of 0 under the mentioned key.
You can also create a registration file to perform the required changes. Just add the following registry code to a file with the name DisableCortana.reg. Once the file has been created then right click on the file name and left click...