what AV did you use before installing avast?..did your computer come with a factory installed trail AV ?
have you uninstalled this?
did you run the vendors removal tool to clear any leftover file that may conflict?

if not you find the tool here. http://singularlabs.com/uninstallers/security-software/

I even used the tool to remove all of Avast and then preformed another fresh install.
did you run avast removal tool from safe mode?.....and reboot