Kelly, if you have Windows 98\Me, the change of the login settings is normal while using avast. See http://forum.avast.com/index.php?action=display;board=2;threadid=7913;start=0#msg64228
With Windows 2k\XP, avast 4.5 has a new mail detection that will work automatically. Please, set up your email account just as avast does not exists. It will detect the pop3/smpt/imap ports and scan your messages.
Hope this helps.