Fail on starting up

Hello Claudio,
Let's try with reinstallation and check if that helps. Use AVG Clear and Remover tool to uninstall AVG.
Refer the below article to download removers for uninstallation:
http://support.avg.com/SupportArticleView?urlname=How-to-troubleshoot-issues-after-upgrade-to-the-latest-AVG-antivirus-product and reinstall AVG Ultimate as suggested here: https://support.avg.com/SupportArticleView?urlname=Download-and-install-AVG-Ultimate

0xC0070643: Error general (0x0643)
What could I do?

Hello Claudio,
Let's try with reinstallation and check if that helps. Use AVG Clear and Remover tool to uninstall AVG.
Refer the below article to download removers for uninstallation:
http://support.avg.com/SupportArticleView?urlname=How-to-troubleshoot-issues-after-upgrade-to-the-latest-AVG-antivirus-product and reinstall AVG Ultimate as suggested here: https://support.avg.com/SupportArticleView?urlname=Download-and-install-AVG-Ultimate