Please check this sticky thread at AVG Free forums about complete AVG reinstallation:
If the installation error 0xC0070643 still persists, please check this How-To article and provide us with installation logs for closer analysis (upload them to online storage and post link to them):
I used to have the AVG 8.0 Free Edition, which was upgraded to 9.0 on my Windows Vista operating system, on my Dell Inspiron 1520 Laptop. Just recently I saw the opportunity of upgrading to the 11.0 version. When I did this it erased my previous version and left me with nothing. Whenever I try to reinstall, it goes through the throes of trying to do it, but then comes up with an error message - installation cancelled due to a higher version being installed. The next message is an unrecoverable error occurred, severity f, error code: 0XC0070643, etc.
How can I resolve this issue please, as I now have no protection?