I think my computer has become a zombie

Last night several e-mails were sent using my AOL account.  They show in the "sent" history in AOL so I do not think it was a case of spoofing.  I run NIS 2010 on the computer.  My updates and full scans were run just the day before.   I have run full scans again and show nothing.  Any suggestions as to where to look? 

Appreciate the help.