can firewalls protect against viruses ?
A firewall is just a part of the protection you should have on your system. A virus is just one kind of malware. There is also trojans, worms, adware, spyware etc etc.
Simply said: A firewall is like a doorman at a bar. He decides who may go in and who not. A good firewall also decides who may go out and who not. So yes, a firewall can stop certain malware from entering/leaving your system and therefor is part of the protection everyone should have. But like a doorman, the firewall only can prevent entering/leaving. Nothing more.
Just curious. It is my understanding that you already had Avast up and running (correct me if I'm wrong) Avast already is antivirus/email scanner. So only thing you had to pick up was a firewall. Than why the purchase of this whole package and which antivirus are you now gonna use?