I suspect someone has hacked your password somehow and been sending bot emails using your PC as a host. try changing your password for your email account. using a stronger password with both upper and lower case letters, numbers and special characters as well. #,!, or @. that stopped my problem when it happend to me.
I have an MCSA cert. I have been in the IT/internet area for over 9 years. I have had many experiences in the IT field and have been in many different areas of the field. I am no longer a Comcast employee and will gladly tell it like it is to anyone. If you are not happy with your service, there are many other providers out there.
Download a firewall or service like spybot you can use trend micro housecall an internet based antivirus sweep to eliminate any of the files that may be a virus or phishing. Personally I use AVG free edition for antivirus which I suggest after using housecall. I also use CCleaner and clean the files and registry every other day to keep the computer clean.
Commercial service and installation technician, been with the company 8 years doing technical troubleshooting and installations for residential and commercial accounts. Im a front line tech and get to see more information of problems than people sitting behind a desk using scripts to help you... Follow me i can help.
How many emails do you have with comcast , are they all doing the same or just the one, You could try as mentioned above by changing the password and security question , Also look in your Drafts folder and make sure that there is no emails in there trying to send something...