There have been some occasions where downgrading to the home doesn't go completely to plan, so it may be better to do a clean install, uninstall, reboot, run the Uninstall Utility (see below), reboot, install (latest version), reboot.
Download the latest version of avast
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall. Register for the Home version
avast! 4 Home Registration form and you will receive an email from register @ avast.com (without the spaces) with your registration key.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.