Pop Up Error Message For Mail Shield

[font=times new roman]Can somebody tell me why I keep getting a message popping up intermittently saying that there’s a problem with the Avast Mail Shield? How do I fix it? I am using Thunderbird. Not sure where I am supposed to put your certificate. The below for the message I get

http://C:\Users\Darian Harder\Desktop

http://www.avast.com/faq.php?article=AVKB91