You have to use another port for avast, e.g. 26: insert this line
SmtpListen=127.0.0.1:26
to file C:\Program Files\ALWIL Software\Avast4\Data\Avast4.ini into section [MailScanner].
Then change the port value also in Outlook Express in the accounts that are protected by avast (menu Tools → Accounts → Properties → Advanced).