a little spambot, I would think. And I'd imagine they are being blocked by either your ISP, or the providers of the intended recipients.
What these things do is harvest the entire contents of your address book and send spam emails. There is usually an attachment to the mail that when clicked on installs the malware in the computer, and it replicates. It's like something out of a horror movie, at a cyber level.
You need to: (1) get yourself some good antimalware programs- evidently you haven't any right now- and scan for the beast and kill it. (2) Contact your ISP and tell them the sorry story, (3) Contact all your address book contacts and tell them not to open it, with reasons. Full disclosure. Sorry, but it's like getting an STD.