Hi and welcome
This is easier then you may think. First save the avast License File You received (in the Registration email sent to you) to a usb
If the Desk top was upgraded then uninstall The pro version using control panel and then reinstall the free version.
Download and install the Avast PRo latest version on laptop and the when asked if you ‘already have a license’ click Browse, go to usb and click on your avast license file.
Here: http://forum.avast.com/index.php?topic=142936.0
You should be good to go. 