The single e-mail was addressed/copied to several people in the sender's Group. Their addressess could, of course, be seen in the received copy. These are what appeared as additions to the Address Book without our manually adding them.
We use ZoneAlarm firewall and Avast antivirus in Outlook express.
Did you reply to this e-mail? and did you use "reply" or "reply all"? If you did check Tools>Options>send and see if there is a check mark in the box "Automatically put people I reply to in my address book". If there is click on the box to remove it.