Windows XP, Windows Vista, Windows 7 and Windows 8 include the shutdown.exe command-line tool that can be used for shutting down or rebooting a system within a script or just from the command prompt. WinPE however is only a minimal operating system primarily designed to act as a preinstallation or recovery environment.
To keep the footprint of WinPE as small as possible many services or tools usually found within a full Windows installation are not available within WinPE. The same applies for the shutdown.exe command.
So how to shutdown or reboot WinPE from the command prompt? WinPE does have a number of command-line tools included and one of them is Wpeutil providing various commands.
To shutdown a WinPE session run:
To Reboot a WinPE session run:
More details on the WinPE command-line tools can be found here