1- uninstall it with any Avast uninstall utility: http://www.avast.com/uninstall-utility
2- reboot in safe mode and scan with MBAM,
3- reboot normally, clean the registry leftovers with Ccleaner (dont forget to backup the registry base)
4- reboot, disable any active security softwares. (windows defender, etc...EAM flagged one Avast file as malicious)
5- Install it calmly and read before clicking next. let avast finish its installation.
5- reboot again.