You computer most likely has an e-mail worm (a type of virus). Someone is now using your computer as an illicit e-mail server for their profit.
Do a full system scan with your anti-virus software. With your anti-spyware as well if you have one.
Quarantine whatever they find.
Good luck.
Use Malwarebytes' Anti-malware or Superantispyware:
http://www.malwarebytes.org/
http://www.superantispyware.com/download.html
These should receive rid of your spyware that's automatically sending emails to your contacts.