Jhiker,
Just an explanation:
In your original configuration (as it seems to me), your mail was first observed by K9, and then scanned by Avast!. Hence the setting 127.0.0.1, incoming port: 110, directed the mail to Avast scanner, which in turns downloads the mail from K9 (localhost:9999 must have been specified in avast4.ini file).
With version 4.5, every outgoing connection is automaticaly scanned, so is the outgoing connection from K9 to mail server.
Outlook Express -> Avast directly -> K9 via default server -> Avast via automatic scan -> Tiscali POP
The correct way to remove the double-scan is indeed directing your mail account to K9 (127.0.0.1, port 9999 in Outlook Express) and let the automatic pop3 scan feature check connections as they leave your PC.
Outlook Express -> K9 (127.0.0.1:9999) -> Avast via automatic scan -> Tiscali POP.
As an extra benefit is the fact that also the K9 program (which is a mail client of its sort) is now protected against viruses.