Most spam is sent out with false "from" addresses. These address can be obtained by using a list of acquired email addresses, or it could be the result of a virus on somebody's system that sends messages out with "from" addresses found in the infected system's addressbook. Either way, this is called "spoofing" the email address.
Lots of the crap I'm getting these days appear to both be "from" and "to" my email address.
While it is possible that you have a virus that is sending out the mail you're asking about, it is much more likely that that mail had its origin somewhere other than on your system. (If it actually had come from your system due to a virus, it probably would have used some address other than yours, found in your addressbook, to make its origin harder to trace.)
When one of those bogus emails gets sent to an invalid address, the server at the other end bounces it back to what the message indicates was the sender -- in this case you -- even though that person really had nothing to do with the original message.
There's nothing you can do about it. Just delete those returned messages and don't worry about them. DO NOT open any attachments.
But... DO make sure you have a good antivirus program, keep it up to date, and scan your system just to be sure you're "clean".
The server at my mail service does a great job of identifying spam mail and marking it as such, as well as catching and cleaning infected messages. Also, I use Mozilla Thunderbird as my email client, and it catches the few spam messages my server doesn't. Plus I scan all incoming mail with AVG and it catches an occasional infection that got thru my server unscathed. In addition, my Thunderbird will not execute any attachments unless I tell it to -- which I do NOT unless I've made sure it is clean first.