Outlook 2000 does not have autocmplete.
Versions of Outlook earlier than Outlook 2002 have a feature called
auto-resolution. If you enter part of a name or a nickname then tab out of
the field (or hit Alt-K [Check Names]), Outlook will attempt to resolve the
name against your address book. If it can't it'll red underline it and you
can right click on the name to manually resolve it. Outlook will save the
resolution for future use.