Avast! has no own firewall (not yet) so you should/can use an other software firewall...
i personally can recommend Comodo Firewall Pro:
http://www.comodo.comi think its not very difficult to use and i think it doesn't need much resources...
but unfortunately its not in German :'( (I'm from Austria)
if you have got a hardware firewall (in your router or so...) a software firewall is not really necessary (people say) - i although use Comodo...
yours
onlysomeone