To encrypt the drive that Windows is installed on, your computer must have two partitions: a system partition (which contains the files needed to start your computer) and an operating system partition (which contains Windows). The operating system partition will be encrypted and the system partition will remain unencrypted so your computer can start.
In previous versions of Windows, you might have had to manually create these partitions. In this version of Windows, these partitions are created automatically. If your computer does not have a system partition, the BitLocker wizard will create one for you using 200 MB of available disk space. The system partition will not be assigned a drive letter and will not show up in the Computer folder.