I helped a friend set up Avast desktop, and he's having a similar problem. He's a bit of a luddite, and I have yet to see this for myself, but he did report this same error message, coming up about 19 times.
His PC is brand new, and I set him up with Avast, Thunderbird, Firefox, and Ad-Aware. He's only been using it for a couple days. It seems much more likely to me that there is some kind of configuration error, than malware, at play here.
I did set up an Avast SMTP alert to send me an email when viruses are found, so it's possible that is what is causing the problem? I tested it and it worked at the time, but maybe there is something I didn't think of.
-Pete