Email account hijacked -- Help!

This evening I’m hearing from nearly everyone in my address book, and not for a good reason. It seems that one of my email accounts (the primary one, unfortunately) has been somehow hijacked and used to send spam to ALL of my address book entries. I’ve read (just a little) about this in the past, so I understand that it is happening (and I see the damn evidence in the return mail that I’m getting!), but I have no idea what to do about it.

I’ve written to abuse@yahoo.com (it’s a Yahoo! mail account), and I’m starting a virus scan of the whole computer. What else can I do? It seems like the whole attack has happened within the past four or five hours. This is embarrassing and upsetting—but I just want to get it fixed.

Thanks