(Joe Rosberg, TechRepublic) Most of us have seen those spoof e-mails, when a personal e-mail address has been commandeered for the purpose of sending spam, but in this case, to everyone in your Address Book.

Here are a few ways it could happen:
Malware of some sort found its way onto your computer, and its sole purpose is to harvest e-mail addresses, which are then sent along to someone else for the purpose of sending spam e-mails.
Someone who has your e-mail address in their Address Book actually has the malware on their computer.
Some Web sites actually harvest e-mail addresses from a computer, especially those that presume to share things with others or invite friends, and so on; or perhaps people who are members of those sites have ways to harvest e-mail addresses from their friends.

What to do:
Scan your system for malware. Two tools I might recommend are Malwarebites and Hijackthis. And since some malware might resurrect itself through a Registry entry, perhaps running CCleaner would be prudent as well. However, consider the risks of running a Registry cleaner.
Make sure your antivirus software is installed and is up to date with the current virus definitions.
Make sure your Windows OS is current with all security updates.
Be careful of (or avoid) some (or all) of those social Web sites, especially ones that share e-mail addresses.
If your computer is clean, and you’re certain you weren’t compromised at a social networking site, send an e-mail to all the people in your Address Book to give them a heads-up that someone in your e-mail circle might be compromised. I would suggest sending them one at a time or with a blind CC, however, since I advise people to never send mass e-mails — although we probably all do it from time to time in certain cases.