Cheers oldfogy & UncleUB.
That October date more or less coincides with the start of the problem (10/11/09) when the auto update was first attempted.
The fact that on that date the Event Viewer logged : "Windows Servicing identified that package KB974470 (Security Update) is not applicable for this system" is confusing me.
I've done extensive searches on the various error messages and it seems quite a common occurrence when Vista auto updating is invoked.
The only explanation that I keep seeing is that .dll files have been corrupted and that prevents the update. But nowhere does it explain how or why those files become corrupted.
As regards that link showing it's not listed for Vista, there are several .NET Framework 2.0 downloads available, so I thought perhaps there is another which is listed for Vista......but searches revealed nothing obvious.
This is why I'm thinking a full re-install is the only solution (confirmed in many searches) which is really annoying when the problem stems from a simple, routine Windows update