Well the windows firewall doesn't have outbound protection enabled by default (Vista, win7), so unless you have Vista or win7 and outbound protection enabled that shouldn't be the reason.
I don't know what version of avast (build number) was installed previously but it may be best to do a clean reinstall of avast.
- Download the latest version of avast, 6.0.1203
http://files.avast.com/iavs5x/setup_av_pro.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear.exe
find it here and save it to your HDD (it has uninstall tools for both 5.x and 6.0.x).
- 1. Now uninstall avast! (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 5.x if previously installed and then for 6.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.