if you want to encrypt your hard drive in windows 8.1 then follow some instruction Swipe in from the right edge of the screen, and then tap Search.
(If you're using a mouse, point to the lower-right corner of the screen, move the mouse pointer up, and then click Search.)
In the Search box, type "encryption" then hit enter .
"Windows 8.1 will automatically encrypt the storage on modern Windows PCs. This will help protect your files in case someone steals your laptop and tries to get at them, but it has important ramifications for data recovery.
Previously, “BitLocker” was available on Professional and Enterprise editions of Windows, while “Device Encryption” was available on Windows RT and Windows Phone. Device encryption is included with all editions of Windows 8.1 — and it’s on by default."