Ignore the email from your ISP as it is probably spam and not from them. Contact them about it.
You do not need anti-malware software on your Mac. I suggest you uninstall Sophos as it will just slow down the computer.
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.
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.)