Avast is most probably the fastest antivirus one can afford currently (both the Home and Pro versions), but it's synergies with quite a few firewalls are not terribly happy.
I do know it works well with Kerio 2.1.5, 3 beta 6, Sygate pro 5.X and Look'n'Stop 2.1.4, but it does not work very well with Kerio 4.0.X, Zone Alarm and Blackice.
I would pick up Look'n'Stop (exhellent firewall albeit commercial in it's full version, with a very tiny footprint), but unfortunately it does not seem to be able to coop well with PPPoE devices.