Never open an attachment unless you are expecting someone to send you an attachment.
Today's viruses have the ability to "spoof" the "From" and "Subject" fields of an e-mail message which prevents you from knowing who really sent you the virus or the content of the e-mail. A virus goes into the address book of the newly infected computer, grabs an e-mail address at random and puts it in the "from field" of a new message. It then attaches a virus to the message, and sends the message to everyone in the e-mail address book of the newly infected computer. There is no telling who sent you the virus. Familiar names are now a dangerous decoy, luring you into a false sense of security and tempting you to open infected e-mail.
If you are receiving e-mail from people, servers, or auto responders and you did not send an e-mail to generate a response, someone who has your e-mail address in their e-mail address book has been infected by a virus and your name was selected at random and placed in the "From" field of the e-mail launched from their infected computer. The e-mail sent from their computer triggered the response which is being sent not to the person who sent the e-mail, but to the person in the "From" field.
There is nothing you can do to prevent the above, but you can protect yourself. Keep your virus protection software updated with the latest virus definitions. I have my computer set to scan on a regular basis and use the "Live Update" feature of my Norton Anti virus software. Run a complete virus scan if you ever suspect that you have a virus.
If your e-mail address is in the e-mail address books of others, there is a good chance that someday your e-mail address will appear in the "From" field of an infected e-mail and some of the recipients who do not understand the nature of today's e-mail viruses will think you sent them a virus. You can also count on receiving lots of e-mail you didn't ask for since your name appeared in the from field.