It's not a good idea to downgrade... Which problems are you facing?
It's not a good idea to downgrade... Anyway, I suggest an installation from the scratch:
1. Uninstall avast from Control Panel first.
2. Boot.
3. Use Avast Uninstall for complete uninstallation.
4. Boot.
5. Stay off-line (not connected to Internet)
6. Install again the old version:
http://filehippo.com/download_avast_antivirus/7. Boot.
Set the program updates to manual.
Go online.
8. Register avast or insert the license file.
Will it work?