To use BitLocker in Vista, you had to create a separate active drive partition.  When Vista first shipped this had to be created manually, but Microsoft released a BitLocker Drive Preparation Tool later to help with the partitioning.

With Windows 7 this partition is still required but is created automatically when BitLocker is enabled.  However the Drive Preparation Tool is still supported as a command line tool, intended mostly for scripting the BitLocker setup for multiple systems.

More information is available at