First there isn't an avast 7.1 version, it is avast 7.0.1466.
Second the avast free version doesn't have a firewall so there is no conflict with any firewall as such.
Do you have outbound protection enabled in the Vista Firewall ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security software (anti-spyware, firewall, etc.) do you have installed ?