That was a good point that I forgot to mention!
I did run removal programs from this site for AVG and Comodo (which I had tried previously):
http://thewebatom.net/uninstallers/security-software/But the firewall still would not operate.
I then installed the fresh version of the software. It seemed like it thought I was still using the free antivirus version, rather than the internet security version, even though it liked the new license (allowed me to at least see the firewall page). I ran update from the add/remove programs in control panel, and it did a fresh install on top of the old one, and the firewall then worked.
Oddly, "repair" in add/remove programs would not run on avast.
Another note: the windows firewall can be on or off, that was not the problem when installing the software. Avast automatically turns the windows firewall off.
I am hoping that Avast continues to support windows xp 64 bit with this software long into the future!!!