Your computer system may be infected with an e-mail virus. This is usually referred to as spam mail, since neither the sender or receiver requested this information. The only way to eliminate this is by downloading an anti-virus software. If you already have one installed, simply perform an extensive system scan to eliminate or quarantine the virus. If this does not work, there are chances that someone has emulated an e-mail program via CGI script. This allows hackers to send e-mails using your account or through a similar e-mail address. You would then need to contact your Internet service provider and they should be able to assist you. Good luck!
The e-mails are probably not actually coming from you. Somebody's probably using your e-mail address and name in the "from" field to hide the real from address. Because you say the sends are targeted to people who are likely in your address book then it's possible someone has gained access to your address book online by guessing your password (so change your passwords asap) or you have a virus on your PC.