After upgrading both the motherboard in my computer and my copy of AVG Antivirus Free there is an error message stating that AVG needs my licence number in order to work.

As this was the free version I never received such a number. I have tried uninstalling AVG and then reinstalling it but this does not solve the problem.
Hi102b, Computeractive forum

The problem here is that some information was left on the computer after the uninstallation, preventing it from fixing the problem. This information is in the Registry so you must edit this in order to fix the problem.

The Registry holds most of the important information about the way your computer works so you should be careful when making changes to it, especially because all changes are applied immediately and there is no facility to undo. Novices should not attempt this tip.

As long as you follow our instructions exactly there should be no problems, but we recommend that you back up all of your documents and emails and make a system restore point just in case.

Left-click on the Start button and then on Run. Type ‘regedit’ without the quotation marks and press Enter.

Click on the small plus button next to the text HKEY_LOCAL_ MACHINE and then SOFTWARE, Microsoft, Windows, Current-Version and then Uninstall. Scroll down the list until you get to AVG7Uninstall.

Right-click on this text and select Permissions from the options. Give your username and the Administrators full permissions and apply. When it asks if you want to apply the same permissions for subkeys, say yes.

If the Administrator or your username is missing, click on the Add button in the Groups or user names area. Type Administrators and then click on the Check Names button. Click on the OK button.

Highlight Administrators and then click to place a tick in the Full Control option. Click on the Apply button and select Yes if it asks to apply it to subkeys.

If it does not ask to apply to subkeys, click on Advanced and select it there. Repeat this process for your username.

Once this has been done, delete the key by left-clicking on it once and then pressing the Del button.

To avoid this problem in the future, uninstall the old version of AVG by running the installation file of the latest version and select the uninstall option.

This should remove the offending key. Run the setup program again to install the software.