Well MBAM (even the Pro version) shouldn’t get in the way as I don’t have any issue with it.

The windows firewall (shouldn’t be an issue, the XP one has Zero outbound checking and the Vista, win7 firewalls have outbound protection, but it is disabled by default.

You didn’t answer the question about other AVs on this system ???

Try a repair of avast:
XP - Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.

Vista, win7 - Control Panel, Programs & Features, uninstall a program, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.

You may need to reboot after the repair.