With the build 9926, Microsoft has made available Cortana for Windows 10 Technical Preview users (in U.S only), she lets you search the web and for files, apps and settings on your computer (Win10). You need to sign into Microsoft account to turn it on. The search box/search icon needs to be visible on the taskbar for you to use the Personal assistant.
After activating it, some users reported it is using precious system resources even when it is not running, if you don’t want to speak to the computer using Cortana, you can disable it, here is how.
Disabling Cortana on Windows 10 Taskbar in build 9926
Right click on the taskbar, hover over the Search and select ‘disabled’ (check the screenshot below).
To get her back, enable the Search box or Search icon.