Hi sanders,
Do you mean you have installed "invalid" avast certificate in your system before?
Not sure. I used to use 7.x and have upgraded to 8 using it's own program update facility, not the exe installer. I don't know if 7.x uses an installed certificate to handle SSL, just had no problems with it and never looked into the certificate store.
Please could you describe step by step what you did to fix the issue?
Unfortunately it is not fixed yet as this currently is not a real security issue for me. I'm going to try installing it cleanly.
Btw, I don't think the certificate is unique for each installation. To be trusted by Windows by default it has to be issued and signed by a trusted certification authority, you cannot generate certificates yourself, that is the key point of certification.
Best regards - Serge.