Hello Graham,
I apologize for the inconvenience. It seems that AVG Program got corrupted when trying to update the files. I would request you to follow the steps in this article http://support.avg.com/SupportArticleView?urlname=How-to-troubleshoot-issues-after-upgrade-to-the-latest-AVG-antivirus-product to reinstall AVG Internet Security by running AVG Remover and AVG Clear tool. These tools will help us to remove the traces completely.
If you face any difficulties, please provide the screenshot of the error message to assist further.
AVG on Windows 10 will not start and shows a window that says "avgui.exe - Entry Point Not Found - The procedure entry point ?GetProductInternalName@AvChromeApp @@UAEPB_WXZ could not be located in the dynamic link library C:\ProgramFiles\AVG\Antiviris\AVGUI.exe"
Graham, If applicable… For your info, just in case that you are unaware, you can post the screenshot (http://avgclick.me/getscreenshot) here in your topic. Click on 'Answer' & then click on the 'Image' [mountain symbol] & follow the instructions.
AVG Guru