It certainly wouldn't hurt to do a clean install.
Download the avast! Uninstall Utility,
find it here and save it to your HDD so you can find it later.
Then uninstall, reboot, run the uninstall utility, reboot, install, reboot.
If you haven't got it already it would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.