William,
Yes, the latest version of AVG Antivirus free is compatible with the Cumulative Update for Windows 10 Version 1709 (KB4056892). To check the version of AVG you have installed, open Internet Security -> Menu -> About -> check whether you have 17.9.3040.
For further help, refer this article: http://support.avg.com/SupportArticleView?urlname=AVG-Meltdown-Spectre-vulnerabilities .
Is AVG Free Antivirus compatible with MS updates relating to the Spectre/Meltdown problem?
I understand that a certain registry key needs to be set.
I think someone is deliberately telling you to ignore me