What kind of spam is it generating? Are they viral? Most viruses now replicate and spoof the sender's address in the process, so that the emails appear to be coming from friends. If this is the case, you may not be the infected one (I'm NOT saying you shouldn't still scan and verify either way), but someone who has you on their contact list may be infected.
It's worth a try to send out a quick email to warn your friends to make sure they have updated antivirus installed and have performed a full system scan. This is always a good warning.
