Win XP - A/V program

I know that I shouldn't be using Win XP but this is for a seldom used backup computer. I tried to upgrade to Win 7 but the Win 7 Win update process always consumed almost 100% of CPU resources on this ancient single core CPU. Therefore, I've reinstalled Win XP, SP2, SP3 and allowed the automatic update process to install all available updates.

I'm still running into may problems.

1. I can't seem to install an A/V program. Sometimes, the downloaded installation program simply will not start. Other times I get the message "can't install because your computer doesn't support SSE2 installation set". Before attempting to upgrade to Win 7, I previously had Avast installed and working. After the Win XP reinatall, I tried AVG, Avast, Avira and Windows essentials. All fail to install. I've also Googled for "A/V programs for Win XP and tried the recommendations without success. Does anyone know how I can get an A/V program installed.
2. I also am unable to get any browser other than IE8 installed. I've tried Chrome, Firefox and Opera. Any recommendations?
3. Some (but not all) HTTPS sites will not open. I ran Windows Diagnostics and was informed that it was probably the Win firewall blocking sites. I turned off the firewall but still have the same problem. Any suggestions?

 

https://www.mozilla.org/en-US/firefox/new/

firefox version 52 ESR is all that will run on xp

I use 360 Total Security Essentials for my av::

https://www.360totalsecurity.com/en/features/360-total-security-essential/