These types of scams/spamming e-mails are becoming extremely common these days and is probably caused by someone else accessing your PC from a remote location.
E-mail addresses are being taken over/accessed by two main ways: (1) by users unknowingly having their password stolen from a fake login page or from web sites that users have signed up on with a password matching their e-mail password and (2) weak passwords that are easily guessed by computers constantly trying to gain access to an e-mail account. Accounts are also compromised if the user responds to any emails requesting personal details like the account password , answer to security questions, etc.
Spammers are desperate for 'fresh' accounts like yours because the chance that it will get flagged as 'spam' (at first) is very low, versus most spam that never makes it to a user's inbox.
Here is what you need to do right away: Please change your password right away to avoid having this occur in the future. If you need assistance with changing your password , please reply and let me know.
Once you change your password , the spammer who is accessing your account from a different location will no longer have access to it and you will be fine. However, please use a different password from now on for your e-mail versus any other web site, and use at least one uppercase letter, one lowercase letter, and a number, like Spectacular10, or Amazing1900. Avoid first/last names, pet names, DOB, anniversaries, etc. Those are much too common.
To restore your address book, please go to "CONTACTS" on Yahoo Mail and select "Deleted Contacts" on the left side and see if you can restore them there. (Older mailbox styles will see “View All Contacts” above the contacts list – if you see this, click on it and pick “Deleted Contacts” and restore them from there.)
Otherwise, if you don’t see them in either of the above mentioned locations, please CLICK HERE. (Link: http://help.yahoo.com/l/us/yahoo/contacts/ab_restore.html)