Your email address may have been compromised. This means your email address has been hacked not your computer.
The best solution to fix this is to change your password and security questions.
This has happened to me and my email was sending out over 10 different spam emails a day to all my contacts. I eventually had my account (email) banned because of this saying to get it back I had to prove that I was an actual a person and not a automated email address by answering my security questions and then changing my password. After this I haven't had a problem.
The best way to prevent spam being sent to your contacts is to have 2 emails. A private one where you only share with your contacts and then another one where you use on internet as your login.
The more websites/compitions you enter, use or share your email as a login/contact the greater chance you have of your email being compromised.