Begin forwarded message:
it looks like someone has used my email address to send out spam. Aside from changing my password, do you have any advice about what I should do?
The chances are best that there is someone out there who has your email address in their contacts list on a Windows computer, and that computer has a virus. The virus trawls their address book and sends out spam to everyone, while “spoofing,” i.e. faking, the identity of the sender.
Unfortunately, it would be hard to track down the infected machine. We hear about this kind of situation pretty regularly, and it usually goes away pretty quick. Rest assured, your Mac is not a carrier of this bug.