This check determines whether the built-in Guest account is enabled on the scanned computer.
The Guest account is a built-in account used to log on to a computer running Windows XP, Windows 2000, or Windows NT when a user does not have an account on the computer or domain, or in any of the domains trusted by the computer's domain.
What's New in Security for Windows XP Professional and Windows XP Home Edition (details use of the Guest account in the Windows XP simple sharing model)
Users overview (Microsoft Windows 2000 Advanced Server Documentation)
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