Somebody’s sending spam using my email address as a fake address.

I have Norton antivirus and a dial up connection, so I’m pretty sure it’s not my computer sending these messages all over the world. But since May 6 I’ve received 155 “returned mail”/”undelivered mail” messages from ISPs apparently of all different countries. I tried to identify the ISP through which the messages have been originally sent, but even the human language is a problem, as they appear to be in Belgium or The Netherlands. Does anybody have any idea of what I could do about this, besides just canceling the email address? Thanks.