DavidR
12
I would suggest you try a clean reinstall.
Download the latest version of avast http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility, find it here and save it to your HDD.
Now uninstall, reboot, run the uninstall utility, reboot install the latest version, reboot.