avast is not a firewall, so attacks is not exactly the case here.
Anyway, virus infections will be avoid since from your boot as avast scans all startup items running in memory…
I think you do not need to worry about this.
If you are using a cable allways on connection I think you should be considering a hardware router cum filewall, backed up by a third party software firewall as neither that or XP’s firewall provides outbound protection.
XP’s firewall is better than none at all but something stronger is required and most so if you have an always on internet connection.