Windows requires that the user have the correct privilege (SE_SHUTDOWN_NAME) to be able to shutdown or reboot the computer.
As an administrator you have that privilege, but not as a standard user. As an administrator you can give this privilege to a user:
- Start the local security policy (secpol.msc) manager (in Windows 8 (or newer) Pro or Enterprise version only)
- Go to Local Policies -> User Rights Assignment
- Double-click on "Shutdown the system" and add the user