I would suggest you download the latest Pro version (
http://www.avast.com/eng/download-avast-professional.html) and save it on your hard disk, so you can find it again. You should have received an email with your Pro Licence/Registration Key (did you) ensure you have that email available.
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 Pro Trial version, reboot.
You will now have to enter your Pro license to convert from the Pro Trial version.