Similar happened to me a couple of years ago. NTL shut down my email account because 30,000 emails were sent from my account in one day.
In my case someone had managed to guess/hack my NTL login and send directly though MY pop3 account from THEIR machine.
Changing my password was all that was needed.
But when you set the pop3 account in Outlook, you can specify the Reply Email address to be sending to someone else (i.e.
bradders@your-isp.com) and this is what the 'Reply' button would use. The spammer doesn't have to have any spyware on your pc nor would they need to know your password, only a random email address.
I think that's right! So if your spyware scan reveals nothing then it should be quite safe to continue using internet banking... can't be too careful tho.