If you suddenly find emails sent to everyone in your address book, you have to suspect that on one or more computers where your address book is stored you have had a virus farm your address book and start sending out to the addresses.
Pretty simple to sort locally by detecting and removing the virus, however since it was sending out with those email addresses it's most probably registered them somewhere on the internet as a legitimate email address and you'll forever more be plagued with emails to it. You may be lucky and removing the virus kills it, but it's the luck of the draw.