Outlook 2003 does not use an Address Book.  The "Address Book" is just a
view into your Contacts folders.  What you are describing, however, has
nothing to do with either an address book or your contacts.  It's called the
autocompletion cache.  Outlook remembers the address you use when you send
messages and presents them to you when you start typing in a recipient
field.  It's controlled by the "Suggest names when completing To, Cc, and
Bcc fields" check box on Tools>Options>E-mail Options>Advanced E-mail
Options.  The file containing this information is
%AppData%\Microsoft\Outlook\yourprofile.NK2, where "yourprofile" is the name
of your mail profile.  read about it here:
http://www.ingressor.com/autocompletetips.htm