I don't think there is a password by default, certainly not one set by avast, the only one is user set to stop access to the program settings. I don't know if when using a commercial license if you were also using the ADNM (avast distributed network manager) software, that might have been able to set a password and if so you are still in the same boat.
The only way to resolve this is by uninstalling and reinstalling avast.
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot. If step 1 failed it may be necessary to run this from safe mode, once complete reboot into normal mode.
3. install the latest version, reboot.