Spam-Bot Exploiting My Email -- Please HELP

this may sound odd, but did you try change to a strong password?

Change your password.

I'm no expert on security, but I seriously doubt malware is the cause of this. People distributing malware generally target the much more vulnerable Windows operating system, and I don't see how malware could help to take over your E-mail account; either way, your password is needed. Change your password, and possibly some other security details, and the spam ought to stop.

Though while you're at it, you might want to change any passwords that were stored in an E-mail on that account.

Sounds like you're a victim of this:

http://feeds.arstechnica.com/~r/arstechnica/index/~3/NzaR7UHUhIY

A group has been exploiting A vulnerability in yahoo to steal session cookies. Log out completely to be sure the cookie is invalidated then log in from a clean machine (live CD) and reset your password , Security questions, and double check the alternate/recovery E-mail is set correctly. As long as the session cookie is still valid the miscreants can get into the account.

Also any info in that account must be considered compromised. Any passwords, financial info (card numbers) etc will all need to be changed to prevent problems down the road.

Hope this helps,
Freemor