Is my computer infected or is it a java problem ?

Two days ago, my yahoo email account started sending out unsolicited m emails to people whom I had sent emails to in the past. i ran microsoft security essential and found several malicious infections. i thought they had been cleared since. two days after, while in sleep (but computer on), my account again sent out unsolicited m emails to at least 100 people. The m emails were sent out in "groups", each email to 10 recipients (yahoo email's limit) with a distinct URL in text (e.g.www.hzxf.aboutcd.com). I ran again the anti-virus but the software did not pick up anything. i tried to update my java, but somehow i can't. any idea *WHO* was sending those emails and how i can stop it. Thank you very much.