I found the answer on the SpamPal board (in case you care). Below is an excerpt of the thread. For the full post, go to
http://www.spampal.org.uk/cgi-bin/yabb/YaBB.pl?board=general;action=display;num=1041896191;start=4You need to set up a "chain" of these programs. Normally, Outlook Express (OE) fetches mail from your POP server. After installing Avast, OE fetches mail from Avast which itself fetches from the POP server. After installing SpamPal, OE fetches from SpamPal which fetches from your POP server. You need to set it up so that OE fetches from SpamPal which fetches from Avast which fetches from your POP server.
I advise getting Avast working first, then coming back and trying to get SpamPal working second.
You'll need to have one of these working on port 111, as by default they'll both try to use port 110. I suggest SpamPal. Go into SpamPal's "Server" options tab and change the POP3 port to 111. Now go into OE's Accounts setup; after Avast's installed you should see settings like these:
Login name: fred#pop.spampal.org
Hostname: 127.0.0.1
Add "@127.0.0.1" to the login name and then go to the "Advanced" account configuration tab and change the incoming mail port from 110 to 111.
I've not tried this, but I imagine it might work.
James.