Some good news to share.
After some searching I think I have found a solution, unrelated to Avast.
This involved fixing microsoft update, by running MicrosoftFixit50202.msi tool on the affected XP machine.
I can now access update.microsoft.com on that machine without Avast returning a Revoked Certificate error and blocking access to the website.
NB: That tool is no longer available or supported by Microsoft, but I had a copy of it in archive.