with Avast free do i need peerblock?
Do you mean PeerGuardian? http://phoenixlabs.org/pg2/
Which is your operational system and firewall?
Peerblock is an updated fork of PeerGuardian. Whether or not you need it depends largely on what you are trying to accomplish.
PeerBlock: Can it run aside with Comodo Firewall 4 and Avast 5?