Success - thanks to Russ Valentine's instructions.
I followed the link he supplied:
http://support.microsoft.com/default.aspx?scid=kb;en-us;829918&Product=out2003
- and created a new profile. This new profile did in fact accept input of new
email addresses...so, too, did the default 'user' account (after it did not,
initially).
**NB - make sure that you switch all views (including 'Start' button) to
'classic view', as the instructions are only applicable to that view.
It strikes me that as I was trying to get the default account's Address Book
working, there were several messages that referenced 'Exchange Server' - such
as 'this feature is not available in Exchange Server - see your Network
Administrator'. My guess is that this installation of Office defaults to
Exchange Server setup rather than defaulting to a single user situation.
Good luck, Mal. Thanks for your assistance Russ.