The first thing to do is to update your virus definitions to the latest and
run a scan of your system. However, you may not actually be sending them
messages. A common practice by spammers is to take a known email address and
put it as the from address on spam email. In other words, you may not be
sending the messages to them, or you may have a virus that is sending emails
to your friends.
Do the emails they receive have a virus attached?