Renew my free subscription

Don't know why you need a screenshot but here it is.Desktop with AVG notification icon and AVG subscription UI showing
BTW, the behavior shield has been sporadically turning off since the first time I installed AVG Antivirus Free so I rely on the warning to manually turn it back on, but that is a separate issue that has been ignored for years.

How do I renew my subscription to AVG Antivirus FREE,
Software version 18.8.3071 (build 18.8.4084.409) UI version 1.0.134
I am using this old version because it is the ONLY one that will
work on my Windows XP Pro 64 bit with SP2. (SP3 only works
on a 32 bit system, not a 64 bit system.)
I am unable to renew my FREE subscription using the AVG UI
because it is FREE. When I try, the response I get is to use an
activation code. When I submit my subscription # (activation code),
it says it's for an unsupported product. The product still seems to be
working and actively downloads new virus def's.
So how can renew it and get rid of the
RED X “your system is UNSECURED” WARNING without having
to completely uninstalling it and reinstalling it???

Hello Bob,

We will help you with this.
Could you please share the screenshot of the unsecured warning message to investigate further? 
Refer to this article (https://bit.ly/2Y6RCDP) for the steps to capture the screenshot.
You can post the screenshot here on your topic. Click on 'Answer' & then click on the 'Image' [mountain symbol] & follow the instructions.
Thank you.

Hello Bob,

We will help you with this.
Could you please share the screenshot of the unsecured warning message to investigate further? 
Refer to this article (https://bit.ly/2Y6RCDP) for the steps to capture the screenshot.
You can post the screenshot here on your topic. Click on 'Answer' & then click on the 'Image' [mountain symbol] & follow the instructions.
Thank you.

Thank you for providing us the info, Bob.
You can uninstall AVG Antivirus Free, download the XP version by visiting this link & reinstall AVG program, to re-initiate the program's validity (however it is free).