I know this is an old thread, but I just had this problem and wanted to post my solution in case anyone is still having this problem. Here is how I was able to resolve it...with the understanding that I am using Vista x64 SP1 with Windows Firewall with Advanced Security with outbound connections blocked. I was having two problems after I installed Avast!: I could not update avast; and I could not browse the web.
To resolve these issues, you need to add two outbound rules:
1. Add avast.setup. For me the path is: "%ProgramFiles%\Alwil Software\Avast4\Setup\avast.setup"
2. Add ashWebSv.exe. For me the path is: "%ProgramFiles%\Alwil Software\Avast4\ashWebSv.exe"
Regarding number two, I am guessing (and this is just a guess here) that all web communications in or out (over ports 80, 443, 8080, etc...) pass through the Avast Web Shield...like a proxy. So if ashWebSv.exe is blocked then the result is like blocking your web clients (firefox, IE, etc...)
I hope this solution works for you...it worked for me.