When you finish with aswclear, look into "C:/windows/system32/drivers/". If there are any "asw*" drivers from avast / alwil (aswrdr2.sys for example), then delete them too.
Also manually search for any folders with "alwil*" or "avast*" as part of their names ("Alwil Software" or "Avast Software" or "avast5" for example). Delete them too.
All that while in Windows Safe Mode. Then reboot into Normal Mode and install Avast, and reboot again.