I repaired Avast 17.3.2290, restarted computer - normal, Behaviour Shield was on. Then I was notified that Avast needed to be updated. Clicked on 'update', 2291 was installed, restarted computer - normal, Behaviour Shield was on. Turned computer off then on again - "Your system is not fully protected", Behaviour Shield was not on.
Go to AvastUI | Settings | Components and make sure that Behaviour Shield is green. Restart Windows immediately after making the change and it should/might stick.
That seems to have done the trick......3 more normal boot ups: "Your system is secured" with Behaviour Shield on.

When I checked Components, Behaviour Shield did have a red 'off' cross beside it, which I had to turn on.
Thank you very much to all who guided me through the troubleshooting process.
One thing to mention when I open Avast through the desktop icon or tray icon 'Open Avast user interface' it is abnormally slow to load.......when I open it through the tray icon 'About Avast' it opens immediately?
Other than that all that I've tried / use seems to be working well, thank you Avast, would hate to lose this software and have the nightmare of finding another of the same standard that is compatible with my system......
The turning the shields off and back on that I previously had a problem with in an earlier program version is working perfectly.
I was going to suggest that ffsta and Vladimir_, the other XP users who are having problems, contact Avast directly / submit a ticket, as I was once advised to do here, but after searching it seems that Avast are no longer providing direct support for the free version - is this correct?