time for action, as I can't send this laptop back to its user displaying Avast error messages everytime it boots(!), So I've taken off Avast and put on AVG (free). First time in years I've not recommended Avast to a home user, so a bit sorry, but got to put on something appropriate (and that means no worrying messages!).
One final observation, of the machines I've tried v5 on: the problem laptop is running XP (Home), my laptop with Win7 doesn't have a problem, a desktop running Win7 RC also didn't have this problem (not yet put v5 on any other machines). So is it an XP thing?