This exact situation happened to our puter. It had an email address of someone we knew, it had an attachment. My g/f and I share this puter, we each have our own email. One came to me from her and one came to her from me. Only thing is, we never emailed each other. Turned out it came from someone we both know. We checked the return path on both emails and they both were sent from the same puter. It had an attchment that we downloaded. It did infect our puter. We saw that we had 101 infected files and growing. We installed McAfee and had them all removed and cleaned up. WHat a mess that was, all ove an email. We notified the person who sent it. HIs puter was completely poluted. It took him weeks to fix it. That advice about checking the header like you mention is sound advice, we look to see if the header matches the email sender, if it doesn't and sometimes with an attachment, we delete it, then go to recently deleted mail and permantly delete it. Friends of mine have had this similar problem at their work, cause panic, they had pros come in to fix their entire system.