Thunderbird sent out 100+ scam emails this morning from one of my Gmail accounts.

Question

Thunderbird is my email software. It handles my email account from my ISP (used for private business), plus 2 other Gmail accounts one for family and one for the local camera club.
This morning, some 100+ spam email messages were sent from one of the Gmail accounts, but accessed the address book of all 3 email accounts - most of the names in each address book are only unique to one address book and not duplicated across the other address books.
I have Forefront, Karpersky and Norton running on the notebook, but nothing showing as a problem. And supposedly, Gmail is meant to scan emails before it delivers them.
The scam emails are all similar - one word in the subject, but with a space in the word (ie De livered), a one line message saying I sent you a message, please open it and then a link to open.
Please help.

Answer 1

One anti virus is all any computer can cope with more simply means more false positive and more chance of none of them working.
Now you say Thunderbired sent these mails. Did a message come up when you started Thunderbird asking to send unsent mails (as would happen if they were added to it's outbox while it was closed.)
did you see anything saying Thunderbird was sending mail?
Or is the mail actually from some web site that your logged into using your, say facebook, credentials that has now spammed your contacts list bacause you allowed to list on that web site.