No, they are probably not using your account.
In all probability they are forging your return address into their own mail headers... coz they don't want to get all of the bounces and complaints, they use someone else's valid email -- forging the header is trivial.
This is called a "Joe-Job". There is boom-all you can do about it, other than contacting your ISP to notify them that it is happening, so they don't block your account on the mistaken assumption that the bounces / complaints are valid.
Otherwise, just wait it out -- in a day or a week, they'll move on to sending with someone else's address forged.
Oh McGiord - good luck on that taking legal action thing. The mails are being sent from zombied PC's all over the world, the website that sells the pills or the mortgages is a throwaway site on a server in China that changes every week, with forged registration information, and who will never respond to requests for action. The address to send the money to will be similarly forged identities and based out of a post office box. You are never, ever going to see the spammers in court.
If you do track down the actual seller, they will simply say "We never sent any emails. We contracted out advertising of our products to this other firm, we have no idea how they send it out. They assured us that it was going to opt in subscribers only and we believe them."