Final answer:
PowerShell is not installed by default in Windows Vista and Windows XP; it was included as part of the OS starting from Windows 7 and later.
Step-by-step explanation:
Windows Operating Systems Without PowerShell by Default
The Windows Operating Systems (OSs) where PowerShell is not installed by default are Windows Vista and Windows XP. PowerShell was introduced in 2006 and was made available for these older operating systems through optional updates or separate installations, but it was not included out-of-the-box. Later Windows versions, including Windows 7 and Windows 8, did come with PowerShell included by default. However, it became a much more integral part of Windows OSs starting with Windows 8.1 and subsequent releases, including Windows 10 and Windows Server 2012 onwards, where it is deeply embedded into the system's management framework.