I'm the only user set as Administrator which should allow me install software.

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So why do I get this message?

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In all the months that I've been using the pre-release versions of Windows 10 having installed & re-installed many programs including WinZip, I've never seen this before of been prevented from installing a program.

The only way around the problem was to run the installer as Administrator; something that I've not had to do before.