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I should have been more specific. Has another McAfee anti-virus application or security suite ever been installed on the computer? (McAfee Security Scan Plus is neither.)
Have you contacted McAfee Support and confirmed that Windows Defender is supposed to be available if McAfee Internet Security is installed? (Many AV apps & security suites disable Defender by default.)
[Between you & me, I wouldn't trust the security of a computer that was running that long without a valid AV app installed, even though all Windows Updates appear to be installed.]
~Robear Dyer (PA Bear) MS MVP-Windows Client (Security, Update Services, IE & Mail) since 2002