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Only certain OEM versions of Windows 7 include downgrade rights:
Windows 7 Professional and Windows 7 Ultimate include downgrade rights to Windows Vista Business and Windows Vista Ultimate.
Windows 7 Professional and Windows 7 Ultimate include downgrade rights to Windows XP Professional, Windows XP Tablet PC Edition, or Windows XP x64 Edition.
Other OEM Windows 7 versions (for instance, Windows 7 Home Basic and Windows 7 Home Premium) do not include downgrade rights.