Potential virus/spyware on my mac?

I have a MBP running Leopard 10.5.6. This morning, I had dozens of emails of returned mail from the emails of various friends in my address book, and had noticed that during the night, my email account appears to have sent a "spam" email to my entire address book (my hotmail account is set up through Mail, which is always running - when my computer isn't asleep). The strange thing is that the lid to my laptop was closed, so my computer would have been asleep - I'm not sure how an email could have been sent during the night. Given that I hear a lot about Macs not having viruses, what is the best way of finding out what happened?

Thanks.

It's not a virus, because none exist. However, there are ways that others can tap into your computer-I don't know how, I just know it's possible.