Well after suffering avastsvc.exe (was avastui.exe before) hogging 50% CPU, I got fed up and removed 18.8.2356 from my WinXP rig.
I then did a clean install of 18.5.2342 (build 18.5.3931.0). Annoyingly that took two attempts.
First time, even when I set Update Program = Manual, it somehow managed to update itself back to 18.8

Second time it correctly asked if I wanted a Security Update, which I correctly rejected to retain 18.5.
Lets see if this version proves to be a bit kinder to my CPU.