Why is people receiving emails I have not sent?

A few people have told me they are getting emails from my email address, they are like adverts advertising viagra.... competitions things like that..... do I have a virus?? even though only recently I took my computer to pc world and they cleared all my viruses and put on Norton anti virus! How can i stop this?? I don't think it is anyone who could of guessed my password or anything?? it doesn't seem like a person sending emails but im not sure????