This has become a common problem in the past couple of months. Please follow the steps below in order:
1. Log on to your Yahoo mail account. Check in the "Sent" folder. If you see the bogus messages in question in that folder, then it can be assumed that someone has hacked your password and is using your account to send spam. You would then need to go to Step 3 (skip 2).
2. If you do NOT see the bogus messages in your "Sent" folder, then your Yahoo account has likely not been hacked, but rather someone else's computer (which does not use Yahoo) is infected with a malware bot that is sending out the messages independently of Yahoo, but spoofing your address as the sender. It gets the addresses to send to from that computer's contacts/address book. If this is the case, then you can't do much but advise your friends of the facts and that you're not responsible for the messages. Then go to Step 3.
3. In either of the above instances, it would be a good idea to first terminate your present Yahoo account, and open a new one with a different e-mail address. Choose a completely new, and hard to guess, password and new "secret questions." Advise your friends and contacts of your new address.
The problem that arises in these instances is that the user doesn't want to have to change their e-mail address, as that causes some inconvenience and is time-consuming. They would rather just change their password and secret questions and leave it at that. IF your situation is as in Step 1 (your account was hacked), then this MAY be sufficient. However, whomever hacked it knows your address and basically who you are, and had access to your address book. Your security is compromised any way you look at it.
The safest and most reliable method is to change your address, and your password/secret questions...and to advise your friends to please make sure their computers are clean of malware.
I strongly suggest using SpyBot Search & Destroy v1.6.2
from Safer Networking for malware protection. It is highly regarded, free, and not too difficult to use.
Hope this is helpful.