The last supported program version for XP is 18.8, so I would guess that is what it is trying to update to ?
I'm only using avast free 18.5 on my XP Pro system as I had issues on anything after version 18.5.
You say it breaks Avast, what do you mean by that, what breaks ?
Did you get sent a new license/subscription number by email ?
If so are you able to copy and past that into your Registration (Avast tray icon) information ?