Like so many (you are definitely not alone) you are confusing Autocomplete
with Autoresolution. Autocomplete was not introduced until OLXP. Make
certain that your contact folders are properly enabled. When you begin
typing a name in the To: field, OL will attempt to resolve that name to your
specified contact folder(s). If it finds a match, it will underline your
typed characters with a green line, no match a red line. This is out of my
area of knowledge, but if you re-post your question asking for
autoresolution help, you'll likely get much better results than I am
providing.
Randy