Hello Ian,
I am sorry to hear that you are unable to reactivate AVG Internet Security in your PC. Don't worry, please follow below steps to get it reactivated with new license number:
1. Please open AVG Internet Security and click on "My AVG" icon at top right corner of it.
2. Then click on "My Subscriptions" icon in it. If you find your old license number there, please click on "Enter new license", enter the new license number and activate it.
3. Otherwise, if you find your email address there, then click on "Log out" option and if you still find old license, then click on "Remove license" option.
4. Please follow the 3rd step until you encounter "Log in" and "Enter license" option. Once you encounter "Enter license" option, please click on it, enter the new license number and complete the activation process.
Keep us informed about the status to assist further.