Yahoo! Mail Virus to Contacts Help.

[KALEL114]

I found out earlier that all of my contacts were sent an e-mail from me with a virus. The email does not show up in my Sent Items list. I have run Norton's and Super Antispyware and they only found tracking cookies. I changed my password, but the e-mails not showing up in my Sent list raised another red flag for me. What am I dealing with here? Any idea on how to prevent it in the future? Thanks in advance.