I had the same problem. I changed \program files\....\data\avast4.ini file like this:
[MailScanner]
Log=100
PopListen=127.0.0.1:111
SmtpListen=127.0.0.1:26
ImapListen=127.0.0.1:144
DefaultPopServer=127.0.0.1:110
DefaultSmtpServer=127.0.0.1:25
Then set Outlook express to use 111 instead of 110 and 26 instead of 25 and voila!
HTH
JGS