Has someone been logging into your account by stolen or guessed password, or does it just seem that your contact list has been copied for spam emails? And do the emails go out to your entire contact list, or just a particular section of them?
If you have no infections on your own computer, it's easily possible that an email you sent out to everyone with your contacts listed (CC) rather than apearing only to each recipient (BCC) was received by someone else who was infected where it was harvested.
My first suspicions (most likely because it is autonomous without having to "call home" to another computer) would be that either your computer or another that you logged onto is sending out the emails.
If you stay logged into an email or IM account in your browser, there are many ways for a website to harvest the contacts or have your computer send out a mass IM or email to everyone.
Another scenerio that is floating around right now:
1.You get an email that says " Joe (someone from your contacts or facebook) has written a message to/about you or his friends at
www.____.com. Please click here or go to
www.______.com to see what Joe has said" Whether you click or go to the site in a new window makes no difference.
2. At
www._______.com, it says to sign into your yahoo/facebook/myspace/etc to see what people are saying about you or your friends are posting. You sign in there and it uses your password to log into your account and send out a message to everyone else to get them to go and give their account details to that same website.
My advice: check your computer and any other computer you use to sign into whatever accounts for malware/spyware/virus. Try changing all your passwords after that to see if it stops.
If your list was already harvested and the emails are coming from somewhere else out in cyberspace, then there's not really much to do other than get a new email and be more careful with it this time. I'd suggest telling people personally about your new address rather than sending out an email addressed to everyone together.
Spoofed emails are almost impossible to trace if done with any skill whatsoever. They don't show who or where they actually came from, they might even show themselves as coming from your ISP or hotmail/yahoo/gmail itself. All it has to do is send the email to a server and claim "I got this email from this other server, and I'm just passing the message along" and it looks legit.