I can also tell you that when I installed Avast, I did disable Nortin Anti Virus software as I was told I can use both.
Whoever told you this did you a disservice, I would advise you don't get your technical advice from. Norton is one of the worst if not the worst when it comes to living with another AV, even when disabled or uninstalled it can still leave remnants to trip other programs up. The SymNRT tool mentioned by Tech should clear any possible remnants.
A hardware modem/router/firewall combo doesn't provide protection against unauthorised outbound connection. Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (sensitive or otherwise, user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so outbound protection is essential.
- Zone Alarm free
http://www.zonelabs.com works fine with avast and has a reasonably friendly user interface. There are others, Comodo, Sunbelt Kerio, Jetico, etc.
See some firewall tests for comparison, some are freeware but many are paid for versions
http://www.firewallleaktester.com/tests.php. Also see
http://www.thefreecountry.com/security/firewalls.shtmlWelcome to the forums.