IMMEDIATELY changing your password for the account and see if the spamming stops.
If someone else has accessed your account it is quite possible that they already have copied your online address book so deleting it might not stop the spamming even if they get locked out of your account.
Read through this thread for additional info -
My hotmail account has been sending spam mail to people in my address book everyday. The best part: My computer is not even turned on when the emails were sent. I have now changed the account password. Can anyone offer me a possible explanation or solution? If it persists, I plan on deleting my address book. Will that even help?