Lately this has been the number one question from clients. The simple answer is you are not using a strong enough password, and someone has figured out your password and logged in as you. Once they have your password they will send e-mail’s from your account and to everyone in your address book. These e-mail’s usually contain a link and download Malware to your computer or try to sell you illegal products.
Click here for a good article from AOL on how to change your password and security questions.