In the Windows 10 Anniversary Update, version 1607, Microsoft removed the on-off switch for Cortana. But the setting is still available if you know where to look.
As with most things Windows, you can remove the search button or box completely if you're really convinced you won't use it. Right-click the taskbar and then click Cortana > Hidden.
With that option set, there's no search box or icon on the taskbar, but you can still access Cortana's full feature set by tapping the Windows key and typing. If you want to turn off all of Cortana's features, leaving only the ability to search for local apps, files, and settings, you need to make a simple registry edit.
On a system running Windows 10 Pro or Enterprise edition, you can use Group Policy to apply this setting across multiple machines in an enterprise network. On a single PC, use the local Group Policy Editor, gpedit.msc, to open the policy Computer Configuration > Administrative Templates > Windows Components > Search > Allow Cortana and set it to Disabled.
On a system running Windows 10 Home, you need to make a manual edit to the registry. Find the key HKLM\Software\Policies\Microsoft\Windows\Windows Search (you might need to create this key if it doesn't exist), and then create the DWORD value AllowCortana and set it to 0.
In either case, this change applies to all user accounts on the PC you're configuring. Sign out and sign back in and you'll notice that the text in the search box has changed from "Ask me anything" to "Search Windows." Clicking in the search box displays only a simple prompt. In addition, Cortana's Notebook is unavailable, only a few settings are available when you click the gear icon, and there's no option to search the web from the search box.
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Next week: Another Windows 10 tip from Ed Bott