Well, we try to be fast.
Let the avast settings as they are (port 110)
Change Spamihilator to listen port 120 and in your email account properties (possibly the ‘Advanced’ tab of the configurations) set the same, 120.

See http://forum.avast.com/index.php?board=9;action=display;threadid=3413 for tons of details about avast and Spamihilator.

Will you come back to AVG? Will you receive this support? ;D