The first time you launch and add an email account in Outlook Express, it knows very little about your contacts: as time passes, though, the email program will start accumulating contacts - by default, it adds to your address book people to whom you reply (prevent Outlook Express from automatically adding people to your address book).
Your address book is also populated when you manually add a contact from an email or create a contact from inside the address book. This information is used to help you send emails in a more efficient manner, sparing you from manually picking email recipients with your mouse every time, or having to type the recipients' full email address. By default, and by using your address book, Outlook Express can suggest as AutoComplete choices email addresses inside the To, Cc, or Bcc fields of the new email window. This tutorial explains how to disable that default behavior and prevent Outlook Express from offering any suggestion.
Do the following to prevent the To / Cc / Bcc fields from offering email address suggestions:

And this is all you need to do to turn off AutoComplete in Outlook Express!
Tip: if you cannot remember a contact's email address when composing a message to him/her, simply click on one of the destination fields' button to insert a recipient from your address book: this works for To, Cc, and Bcc.