I have to clean install windows, will I be able to use my avast internet security license key after I clean install windows and install avast free anti virus to upgrade?
You can use the same license.
You don’t need to first install avast free - download the AIS http://files.avast.com/iavs5x/avast_internet_security_setup.exe version 7.0.1474 direct from avast.
Hopefully you saved the avast! license file and it was not wiped out by the clean install. If not you will have to contact avast! to obtain a copy of your license file