The simple answer is:
The email addresses have been spoofed. A very common practice for spammers. They basically "borrow" someone elses address for a short time, thus making it appear that you sent the messages. Some mail gets through to inboxes, others fail because the address are not valid.
There really isn't anything you can do-just wait it out, as the spammers generally don't use an address for long. It technically isn't spam-what is sent to others in your name is, but the bounce back messages aren't. They come to you because it was sent by someone using your email address.
It is no different than me sending someone a snail mail, and using your home address as the return address.
Of course, infection is a possibility, either on any of your machines or the computer of someone who has you in his/her address book.
Edited by Queen-Evie, 29 April 2008 - 04:46 PM.