Problem has been solved! Everything seems to be ok! 
Yep!! Everything's back to normal... Well done gentlemen! 
It would be quite useful for all concerned if there were some explanation of the cause of this problem.
I have just started up my desktop W7 system, and, for the first time ever, I see in the Event Viewer an Application Error for chrome.exe, which is a prefetch in my startups with this entry:-
Yes HKCU:Run CFA4BAFF5696A87DDBFEBDBB843936345DB9A8D9._service_run Google Inc. "C:\Users\Username\AppData\Local\Google\Chrome\Application\chrome.exe" --type=service /prefetch:8
Thus, the chrome prefetch isn't running on my machine as it would normally be.
I have previously noted in threads on this problem that killing this running entry from Task Manager caused Avast to crash immediately with the error that has bedevilled us recently. I have no idea whether this was just coincidence, or whether the previously-unseen Application Error on chrome.exe I describe above is somehow connected to the Avast fix to solve the UI/Avast service problem.
It would be good to know a bit more about this whole thing. I am never happy with "fixes" which involve fresh installs and so forth, and they remind me of the standard useless Microsoft response whereby the inevitable "shot in the dark" blind recommendation for fixing unexplained problems was a clean reinstallation of Windows!
