malfunction in outlook auto complete

we are running outlook 2000 (not exchange), and my auto-complete function stopped working after I put up a new computer. all office 2000 software was loaded as usual but, certain features dont seem to be consistantly available. One of my staff lost a part of the dialogue box that says “put all peolple i reply to in contacts” and now I have lost the ability to auto-complete- like when addressing emails = i always have to open my contacts and maually put in addresses – does anyone know whats up plz??