Check out this
avast! Video Tutorial - Insert Registration KeyYou shouldn't need the 'antivirus removal tool' uninstall utility, just use the windows add remove programs to uninstall the pro trial version, reboot, install the home version and enter your registration/license key.
You could just enter the Home registration into the pro trial version and providing you are on-line at the time the version would be downgraded to the home version. Though some have reported issues doing this so it is probably better to uninstall, reboot, install the home version.