On October 2 at 1035 PDT a spam e-mail was broadcast to everyone in my Yahoo address book from my Yahoo e-mail address. I access my e-mail from Yahoo's website. I was not connected to Yahoo e-mail at the time. I discovered the incident later that day and immediately changed to a stronger password - previous PW was 6 alphanumeric characters, lower case only. When I reported the incident to Yahoo support, they said it was likely my computer had been infected and that was the cause of the incident. However, I had run Norton Security Scan (Full Scan) with current definitions the week before the incident and it indicated that my computer was clean. I did the same thing after the incident and again my computer was clean. I reported this to Yahoo support, but they believe my computer was infected by the Conficker Worm. Is it possible that my computer could have been infected between those NSS scans and the worm, virus or trojan caused the incident and then removed itself leaving no trace on my computer? I notice that Yahoo e-mail login appears to allow unlimited attempts at password verification. My previous PW would have had 36^6 combinations. Thanks in advance for any help or insight.
[edit: removed email address per the Participation Guidelines and Terms of Service.]