Support for XP ended some considerable time ago, the last version of Avast which supported XP was version 18.8. The Avast link you gave would only serve the latest avast version/s.
Avast doesn't hold old versions for download, so you would have to find version 18.8 from a file download site, such as filepuma.com.
That said some XP users (myself included) experienced issues with 18.8 and had to revert to 18.5.
Now, the next big question is, does Avast still support that version for virus definition update (it might) and if so for how long. I haven't used XP for some considerable time and I just don't know that.