Most spammers hit and run from where they send out their spam. They seek out systems which are unprotected relays. In today's world, this is rare, but they still exist, especially in Asia and South America.
Most of the trojans infecting PC's today are spam emailers - they infect various PCs and either get their information (to/from addresses)from one's outlook address book or they generate it randomly from sets of common usernames and domains. These infected PC's are called Zombies.
Good articles on Zombies:
http://spam.gunters.org/The one thing you should consider is that spammers usually don't stick with any one thing for too long, as system admins update their spam filters, so I would expect this attack to end as quickly as it started.
If you wanted to, you could learn how to read email headers (ugh, but useful) and see if the msgs are coming from one or various sources. If they're from various sources, I'm afraid there's not much to do. If all the messages are coming from the same source, then there are various things you can do:
http://spam.abuse.net/userhelp/howtocomplain.shtmlTutorial on reading headers:
http://www.visualware.com/resources/tutorials/email.html#headers( Don't buy their software - instead, get the freeware program SamSpade:
http://www.samspade.org/ssw/screenshot.html which will look up the same information )