Or they are simply spoofing the return address. From what I've read/heard, it's hardly worth the time to find out where it actually comes from. And by the time you do, the SPAMmer will have moved on. The true source is probably a PC froedn who has not protected his address book/computer. Your address is in there and the SPAMmer has installed a little app that sends emails to everyone in there.
So, two solutions:
1. Keep deleting them
2. Get rid of any friend using a Windows PC
You might also try training your email app/anti-SPAM software by letting the messages come through and making sure you mark any not caught as "spam." As long as you don't actually download them, they will not do any harm. Especially if you have html turned off and don't respond to them. There is still no known virus for the Mac in the wild, although some have been devised.