Are you sure it's the Mac sending out the spam, or just spam masquerading as coming from your email account (the From: line is ridiculously simple to forge)?
While it's clearly possible to devise a system to cause a Mac to send out spam email (or any other computer for that matter), it wouldn't be terribly efficient for the spammer. Even when they take over zombie machines (and unpatched Windows XP systems are the favorites there) the spammer would prefer to use the machine to send out all kinds of email *AND* would attempt not to draw attention to the actual machine sending out the stuff.
That is, where it *appears* to be coming from will generally not be where it is coming from.
What malware prefers to do is "harvest" an address book from a user's computer (or, more often these days, from an online address book after phishing the credentials from the user) and then using that to send out emails to your contacts that look like they come from you so that they are more likely to be opened. If that's what has happened, there's nothing you could do to the Mac at this point to get that data back.
Again, while it is clearly possible to harvest that data under either OSX or Windows 7 from your address book in either platform, it is considerably more difficult to do that than to go after your credentials for an online site using a phishing attack and then make use of the contact information.
If the emails are not in your Mail.app "sent" folder they did not originate from your MacBook.
It is not uncommon for an email server's accounts to be hacked, or for your email recipient's Windows computer to be pressed into service as a spam-bot, substituting your address as the "from" in mail it sends.
Change your passwords and make them non-trivial. Problem solved.