Autocomplete is not working in Outlook 2003 or Outlook 2007
If for some reason the autocomplete isn’t working on Outlook 2003 or Outlook 2007 in the To: CC: or the BCC: fields, then you might want to make sure that the ‘Suggest names while completing to To, CC, BCC fields’ option is enabled. You can find this option in the ‘TOOLS->Options->Preferences -> E-mail Options -> Advanced Email Options
If even with this option checked and Outlook is still not autocompleting, then you might want to rename the *.nk2 file in your profile. You can find this in the ‘C:\Documents and Settings\yourUSERNAME\Application Data\Microsoft\Outlook’ folder. Once you renamed them, restart Outlook and try again. This time it should autocomplete.