I've "updated" to several of the 18.x versions of Avast Free this year.
Avast Free at least through 18.4 has consistently caused problems for me with Windows XP. I was following previous posts in this forum on this topic but did not identify any resolution - although I haven't checked the the topics recently.
After each of the "updates" I've had to reboot in Windows Safe Mode, use the Avast Clear tool to remove Avast, then return to normal mode and re-install Avast 17.9.2322. That's the last reliable version for which I have the full installation package.