Sophos can be slow on the initial scan. However, you shouldn't need to do a full scan. You're almost certainly not infected with anything. Sophos has quite good detection rates, and there are no currently known Mac botnets running.
It's possible that this is simply a false positive. Comcast has a reputation for that sort of thing. Whether it's a deserved reputation or not, I don't know, as I have no experience with them. However, it's not uncommon for Windows-oriented security solutions to throw up false positives on network transmissions sent by Macs, because they are different than how Windows typically behaves.
Alternately, it's possible that someone on your network has a Windows machine that is infected. Are there any Windows machines in your house? If so, go over them with a fine-toothed comb. If not, does your wireless network require a password to join? If not, someone who is infected may be piggybacking on your network. Lock down your wireless network with a good password, and use WPA2 encryption. (The weaker WEP can be cracked in under a minute these days, so you might as well not use any encryption at all if you're using WEP.)