I would say your problem has nothing to do with spam, but rather someone has spoofed your email/ or has opened up a port on your PC to enable them to send mail via your email gateway. Hence you are receiving a lot of undeliverable email. (Check the description on the undeliverable mail, you should be able to establish who the mail was intnded for and the subject, also the time it was sent) Now if you don't recognise it, you have a problem
The most likely cause is a trojan is resident on your PC. and is using your email gateway to send SPAM to other people, in form of advertising/the usual junk. You can get rid of this, by trying to install something like microsoft antispyware (it is free, I have had no problems using it. But I have had it installed since the PC was built) If that dont work I suggest using ad-aware
If someone is spoofing your email address from another terminal, you have no real control over this, apart from reporting it to your ISP or changing your email address