Tech,
Thanks for pointing me to that other Avast thread - I got some more useful info there.
My further researches into the problem behaviour of Windows Security Centre (WSC) and the Repository, rapidly lead me to the Problems with Windows Management Instrumentation (WMI) and then to WMI Diagnostics.
(
http://www.microsoft.com/technet/scriptcenter/topics/help/wmidiag.mspx).
It quickly became clear that I was out of my depth and into potentially dangerous territory. Time not to fix what isn't broken!!
I've installed the MS KB933062 WMI stability update, and will see what happens when next I change antiVirus and/or firewall applications. I.e. Does WSC behave?
If it doesn't, I will then use the aforementioned batch file, which seems to work (for me at least).
So it seems Rodalsa's simple, and reasonable, question gets harder and harder to answer completely.
Maurice