This is not normal spam. That is sent out by outfits to people from a list of E mail addesses or in block, hoping to get a hit.
If in your case they are using your E mail address to send them then you security has been compromised. Are you sure that it shows you as the sender?
Get yourself a firewall and a virus checker and change your E mail address.
It's a pain but needs to be done.
I am sure that you will get a more technical reply from someone soon but this would worry me.