Follow the suggestions I gave, if you have the option to insert the license file, get the free registration key (info in my link) and try that.
As I said I'm not hopeful that it will work, either it won't allow the free registration key as it is expecting the pro license or it won't downgrade successfully. Either of which will mean a clean reinstall is required and that would probably be best to also run the aswclear.exe (uninstall utility after regular uninstall).
- Download the latest version of avast, 7.0.1426
http://files.avast.com/iavs5x/avast_free_antivirus_setup.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 5.x, 6.0.x and 7.0.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 6.x if previously installed and then for 7.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.
Programs & Features, Uninstall/Change