I have answered my own question. To show the Bcc: field when composing an email message in Windows Mail, first click the Create Mail button in the menu bar. A new message form will open. In the New Message menu bar, click View and select All Headers. The Bcc: field will then appear. 
Using Windows Mail v.6 under Microsoft Windows Vista SP2, how do I get the Bcc: (blind carbon copy) field to show when I compose an email message? 
Bcc: field in Windows Mail