Dear unwitting Spammer
Depends what the spam message was...
Recently, a lot of the spamming webites use a different approach than usual.
They send out an email from their servers but using your name in their account names & the email address. The domain name obviously will be different, but this evades many anti-spamming policies running on online email clients like Hotmail, Yahoo etc... Sometimes they even evade the local anti-spamware too as some of these anti-spamware cross-checks the username with your address book & since the name is the same, it lets it get through...
So:
It might not be you...
It might not be your computer...
It might not be your programs...
It might just be clever people out there using their brilliance for the 'not-so-good' purposes....