I did all those things and more for 18.4 without effect (see previous posts).
"Using XP SP3 - no problems for many years with Avast Free Antivirus until this latest - UI does not load. Some postings indicate that fixes have been made, at least for Win10. However, on XP (SP3), version 18.4.3895.0 still has the problem. I have uninstalled, reinstalled, rebooted, and repaired. I have made all the suggested tweaks to "Services" (background intelligent transfer service; terminal services; remote desktop service), but the problem remains."
My XP works flawlessly in all other programs, and has worked on all previous versions of Avast through 18.3.
This is seems to be an issue introduced with 18.4 and has not been addressed in the beta 18.5. As others have noted, it apparently does not occur in some XP installations, but affects others. Probably some minor registry setting?