The free version is all you need to test and run on Windows XP SP3.
See
https://www.avast.com/windows-xp-antivirus
Or
https://www.avg.com/en-au/windows-xp-antivirus#ios
You need Windows XP SP3 for either version and the correct bit version (32 or 64).
You still get virus definitions updated, you just don’t get new features found in versions for newer computers.