Update on this topic from the person who started it:
The solution worked up until recently when Avast removed the repeat registration mechanic which forced me to update to the new version. Problem is, this new version just flat out won't start at all. I've been running it for the best part of 20 minutes now and the task manager shows me AvastSrc.exe is active BUT it neither opens or activates fully.
It does however produce a stunning 3.5 million page faults and a total of 8 minutes of CPU time. So what is the latest version I can revert to that will safely work for this machine and how do I do so?
PS. I still can't update this computer to a newer windows for reasons which I am not at liberty to discuss. Please do not ask.
It's not my primary machine any more but things have become... complicated.