bob,
To be completely safe, run a full Avast scan and throw in an Adaware.
If you have A2 ( a2 or A-Squared anti trojan scanner), run that also.
What appears to be happening (or it did) was that a trojan or worm used your email header to "resend". Spoofing is a pain in the neck as you get blamed for the "trash". If it continues or reoccurs, let your ISP know.
As Rocker has stated, this is common but still must be addressed.
It could be a one time shot, but make sure that no little "bug" was left behind to do it again.
Good luck and Happy Holiday.