It is likley that these emails are, as you say generated by a virus or a worm. However it is at least as likley to be on someone else's machine as yours. i.e. anyone whom you have emailed in the past and hence who's PC "knows" your email address or your subdomain which a bit of malware uses as a "seed" to generate email addresses to mail to, and to fake from addresses.
Check your machine with another antivirus programme just to be sure.