These message come from the virus infected computers and the attachments are usually infected. The virus writer is trying to lure you to open attachment to check out the message you supposedly sent but which has not been delivered. Why do you get those messages? First, this is not a spam, which goal is one way or another to separate you from your money. This is a pure malicious attack, however, it's not personal. Here is how it works. Let's say there are three people, A, B and C. A knows B and C and has their email addresses in his address book. B and C may or may not know each other. A's machine gets infected. The virus extracts email from A's address book and then sends email to B using C's email address as a return address and vica versa. B's antivirus software catches a virus and he writes C that C sent him a virus and threatens him to complain to his ISP, FBI and his mom if C does not stop. C replies telling B to fuck of in general because his machine is clean and antivirus is updated in particular. You get the picture. On the side note, A's machine IP address can be found in the infected email headers.
