Find the Secure Boot option and change it to Disabled. Save the changes and reboot again. We recommend keeping Secure Boot enabled unless you're sure it needs to be disabled. This article explains how ...

As a good PC user, I run security software in Windows to protect my computer from malware that may pop up as I go about my day. But what about malware that tries to launch before Windows even loads?

It sounds to me like claiming that all old Windows versions will be unbootable is an overstatement. Rather, it's most likely the case that you won't be able to boot old media when Secure Boot is ...