Like Windows RT, Windows 8.1 now encrypts your storage drive—maybe. While all Windows RT tablets support encryption, very few PCs are likely to be compatible with Windows 8.1's device encryption.
To support the feature, your PC needs to have a laundry list of not-very-common checkboxes ticked: Secure Boot support, Trusted Platform Module 2.0, and Connected Standby (with all its own obscure requirements).
If your PC supports all that, then you'll find the device encryption in the PC Settings section of the Settings charm. Head to PC and devices > PC info: If your hardware gets the seal of approval, you'll see a device-encryption option. (Mine doesn't, despite the fact that it rolls with a high-end Ivy Bridge Core i5 processor and an SSD. If it did, 'Device encryption' would appear underneath 'Change product key' in the image at left.)