I used microsoft security software, but no avail, my hotmail send out e-mail and I can`t stop it. I check for root kits, malware and trojans, found nothing

I get other people e-mail that meant for someone else, not in my household.  My computer is sending out e-mail to company and retails that I do business,  but I am not the one that sending it out.  I used Microsoft security essentials, Sophos`s root kits and malware- free.  I can`t stop it.  But may think the one link is the e-mail I am get it from is the guy Hotmail.  I think maybe he is the problem or he been hacked and does not know it.  But right now I want to stop it from e-mailing from my computer.