More likely, your address has been forged on some spam, which has bounced.
A lot of spammers seem to cycle through their "victims" addresses as forged sender addresses - it is so trivial to forge an address - even I could do it.
I was really dismayed by this news item on Net4Nowt:
One in five consumers buy software from Spam
Why do spammers keep doing it?
BECAUSE THEY DON'T NEED MANY SUCCESSES FOR IT TO PAY!
http://www.spamdontbuyit.org/ - Spread the word, give spammers the cold shoulder!
- Do you have a firewall?
It's possible you have a trojan,
or somebody on the same ISP with you in their address book does - with a subject of "attach", it sounds more like a virus than a spambot, and the usual ones tend to forge an address from the address book, rather than calling attention to the address they ARE at.