A similar problem to Willem below. AVG was installed and working with Windlws 10, but suddenly would not validate. I followed the advice given to uninstall and reinstall, but get error code 0xC0070643. Windows Defender seems to be switched off, but won't let me change anything anyway. So I have no online protection.
Hello Alan,
We apologize for the inconvenience caused.
Mostly this error is occur due to anyother Antivirus program or previously installed AVG on your system.
Is there any other Antivirus program or AVG program installed on your system?
Have you tried "AVG remover tool" to uninstall and reinstall AVG?
Thank you.