Note: Typical uninstallation from the Control Panel in Windows can be insufficient in some cases. Some records may still remain in the system and prevent avast! from being correctly installed. Most vendors offer special removal tools to uninstall their antivirus software completely from the system. If there is mentioned an option to use removal tool in the instructions linked above, we recommend to use it for proceeding with the uninstallation.

https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=574

http://uninstallers.blogspot.com/