If they appear to come from you, one of your friends most likely has a virus or you do.
Spam is impossible to stop once it gets started.
Use a throwaway email (hotmail or yahoo etc) for signing up to things and a separate one for close friends and family.
They most likely got your email address by somebody passing on one of those silly attachments that people tend to send to everybody in their address book. The email will eventually get to some spammer and sold on to other spammers.
