It's useful, I'll expound a bit below but also echo what Ray (Popowich) said. If it's someone using your email address as the sender but it not actually coming from your account; that's called 'spoofing' (aka Joe Job)
Yes we agree that spoofing is being done. But let's not say joe job. That is something different and will only confuse the OP. See Wikipedia and below.
If mail is going out to your contact list as the original poster suggested, it's very likely that someone has access to the account
No, no, no! The spammer may or may not have access to the account. Only yahoo or hotmail can tell us who is the "compromised" account, or the "access" account, or which account it is "coming from". As Ray says, spammers will spoof so they don't have bounces coming back at their servers chewing up bandwidth. As I say, the spammers will spoof to get the recipients to see a familiar name in the header, which is totally easy for them to do if they have people's address books. Either way, we all agree that spoofing is probably going on.
If so, we are doing a disservice to the OP to get him to think that changing his password will solve the problem. Also, if he has spyware on his machine and his account is the "compromised" account, then the spyware will just read the new password.
Incidentally, if you look at the OP's full headers, you will probably see that the messages originated from foreign countries. I'm not clear how that helps the OP.