My computer had crashed in October - a hacker got into my e-mail address sending out bogus information about?

Viagra and a private part with an attachment under (No Subject). I have had so many of my contacts complaining about this - and I am telling them that I am not sending this mess out. Am afraid it might have a virus in the attachment if someone opens it. How can I stop this and report it and also how to change my email address? Also, is there a way I can find out who tapped into my email address illegally.