Thank you Bob,
The issue with Behavior shield getting turned off automatically was fixed through one of the virus definition update. I understand you are using XP computer with latest AVG 19.3 version. This issue shouldn't be happening in our latest version. We would need to do a complete re-installation of AVG by removing the traces manually. To perform that, I have sent you the steps through email.
I was asked to start a new post from my account to find a solution to this ongoing problem. Please see https://support.avg.com/answers?id=9060N0000000RSE for the complete conversation, but here are some shortened excerpts from that conversation.
March 14, 2019 Bob Jackson
Why has this issue been marked as "Solved" when clearly others have this same problem and no actual solution has been publicly posted here.
March 14, 2019 Karthikeyan (Avast) 
Bob, this issue was reported to be fixed in other posts.
March 14, 2019 Bob Jackson
Thanks Karthikeyan, that's great but please direct me to these "other posts". ... The AVG Antivirus Free I have is Software version: 18.8.3071 (build 18.8.4084.409) UI version 1.0.149
This is the free unpaid version
March 14, 2019 Ranjani Bakthavachalam (Avast)
Bob,
We came across few posts where customers mentioned that the issue has been fixed.
We came across few posts where customers mentioned that the issue has been fixed.
March 15, 2019 Bob Jackson
I use Xp Pro 64. ... Why can't you simply give me the links to the posts where you said "We came across few posts where customers mentioned that the issue has been fixed"
So here we are with a new topic that is said to be already solved and still you have not shared these other posts where customers mentioned that the issue has been fixed. So I'm asking again, please share these posts with me?
Hi Shankara,
That was Friday and now I want to let you and others know what happened over the weekend. On Saturday morning when I turned on the computer, all the AVG shields stayed on and after over a dozen reboots during the entire weekend, all the shields remained on and everything still works.
I’m guessing that the Saturday morning definitions auto-update fixed the problem just as you and others said they should. Fortunately I did not have to do any re-installs at all.
The AVG Antivirus Free version: 18.8.3071 (build 18.8.4084.409) now works on my Windows XP Pro 64 bit SP2 system. I would like to thank you, Ranjani, and the rest of the customer care team for been very patient and helpful. Best regards to you all.
We are glad to know that your issue has been fixed.
For XP, the highest version available is 18.8. You cannot update it to the latest version.
Let us know, if you have any issues.
Thank you Bob,
The issue with Behavior shield getting turned off automatically was fixed through one of the virus definition update. I understand you are using XP computer with latest AVG 19.3 version. This issue shouldn't be happening in our latest version. We would need to do a complete re-installation of AVG by removing the traces manually. To perform that, I have sent you the steps through email.
Hi Shankara,
That was Friday and now I want to let you and others know what happened over the weekend. On Saturday morning when I turned on the computer, all the AVG shields stayed on and after over a dozen reboots during the entire weekend, all the shields remained on and everything still works.
I’m guessing that the Saturday morning definitions auto-update fixed the problem just as you and others said they should. Fortunately I did not have to do any re-installs at all.
The AVG Antivirus Free version: 18.8.3071 (build 18.8.4084.409) now works on my Windows XP Pro 64 bit SP2 system. I would like to thank you, Ranjani, and the rest of the customer care team for been very patient and helpful. Best regards to you all.