try doing a new clean install…

you may remove it using this http://www.avast.com/uninstall-utility

and you have removed any previous used AV program before you installed avast?

Uninstalling third-party antivirus software (Removing your Current AV)
https://support.avast.com/index.php?languageid=1&group=eng&_m=knowledgebase&_a=viewarticle&kbarticleid=574