In a pretty radical departure from previous Windows versions, if your PC hardware meets the trusted platform specifications, and a clean installation of Windows 8.1 is performed (and you login with your Microsoft account, or a domain account), your hard drive will be encrypted by default.
If you have an older Windows computer that you’ve upgraded to Windows 8.1, it may not support Device Encryption. If you log in with a local user account, Device Encryption won’t be enabled. If you upgrade your Windows 8 device to Windows 8.1, you’ll need to enable device encryption as it’s off by default when upgrading.
More details at HowToGeek