I got it working!
For anyone who may stumble on this post in the future:
I followed the directions found at this link to block all outbound connections through windows firewall.
http://www.komando.com/tips/index.aspx?id=2973The document tells you in a very clear way how to create rules to allow outbound programs to go through the firewall. I created a rule for every .exe file that was in the Avast folder in my Program files. I probably didn't need to do it for every one, but it was easier than trying to figure out which ones didn't need it.
Then, I couldn't update. Avast tried to connect to the server, but was being blocked by the firewall. I only knew it was the firewall because it updated fine before I followed the directions to block all outbound connections, as Vista's firewall does not alert when a program attempts to connect and is blocked.
The problem is, that when Avast updates, it needs a specific file called avast.setup to connect to the server. It is located in the "Setup" folder. To create a rule for it, in the rule making wizard, I could not browse to choose the program, because avast.setup is not an .exe file. So, I did what DavidR said to do above: type in the path to the file, instead of navigating to it. That did the trick. I feel somewhat silly for not trying that, but oh well. Live and learn.
Now if only I could figure out how to allow a Windows Update without creating an outbound rule for svchost.exe, which I am told creates a vulnerability.
Many thanks, gents.