Windows 8 now allows the OS drive to be encrypted without TPM and USB.
You can use a password as a protector for OS drive.
You will have to enable the below GPO under BitLocker Drive Encryption/Operating System Drive/Require Additional Authentication at Startup.
Make sure you check “Allow BitLocker without a Compatible TPM (requires a password or a startup key on a USB flash drive).
This will help for machines who are in countries where TPM is not allowed like Russia, China etc..