That link ultimately goes to the regular avast installation version (nothing special about XP SP3), however this only uses the on-line installation file.
As you should note from my signature underneath my post/s I have one system still running Win XP Pro SP3 and the latest avast free version.
You say it will not install - the on-line installation seems to have hiccups from time to time - if you have any internet hiccup then it could corrupt the installation.
I would suggest using the full off-line installation file, see this topic -
https://forum.avast.com/index.php?topic=218904.0 - for the download locations. Choose the avast_free_antivirus_setup.exe option. Although this says it is an off-line installation, you still need the connection as one element that isn't on the file (required by XP) would need to be downloaded during the off-line installation.