In common with many others, I have had my domain names used by spammers as the return address for spam, some including viruses. Such emails are forgeries. The spammer created the 'from' and 'reply-to' fields. I categorically assure you these messages did not come from me. And neither is my computer infected by a virus that is sending them. You can confirm this by looking at the message headers (see tutorial on www.stopspam.org). The headers will show that they do not come from me.
As far as I can tell, there is nothing I can do about this. On other sites people in my position suggest that you send back the offending message. If you wish to do that, please do and I will pursue the culprit. I will completely understand however if you don't, because that is exactly what a spammer would want you to do so as to obtain your email address!
So, what to do? I suggest:
My sincere apologies if you have been affected by this forgery. I am as affronted as you are, I can assure you.