Help with Inbox Hijacking

Hello.
I am running Thunderbird on a Dell laptop with Vista. About 10 days ago something sent a spam to everyone in my Thunderbird address book. I ran a Panda scan and it found 4 things which it disinfected so I thought I was OK. Then two days ago it happened again. I ran another Panda scan which found nothing.

I have uninstalled Thunderbird 2.5 and replaced it with the beta version of Thunderbird 3. I have removed my address book (copied it offline) which I hope will stop whatever is going on from at least making my correspondents crazy.

I ran all the stuff under the malware note and none of it seemed to identify anything (although I can send the reports if need be). What else can I do?

Also, how can I tell if it's fixed other than if nothing else bad happens for a while.

Any help would be greatly appreciated.