There are a number of viruses (Sobig, Klez, Nimda, Mydoom, BugBear, NetSky, etc.) that forge the sender's E-mail address (FROM address) so that they look like they are coming from a different person than the one that actually has the virus. In many cases, this forged E-mail address is yours.
These viruses are infecting the PC of someone you know, and this person has your E-mail address in their address book. What this virus does on an infected machine, is that it tries to spread itself by sending itself to everyone in their address book. It forges the "FROM:" address by picking a random address from the address book (which may be your E-mail address!). So others get E-mail sent to them with your name on it (but you didn't send it, your "friends" infected system sent the mail out). So that's why other systems then reject the mail and bounce it back to you or the recipient claims you sent them a virus.
How to get rid of this?
All people on the Internet must learn to keep their systems up to date, install security patches as needed, and use up to date anti-virus software. Also many ISPs, including Atlantis International, no longer bounce back messages from viruses, because of the high likelihood that the From: address was forged.
For more information visit:
Email Virus - False Alarms
Worms that FORGE email
Email Spoofing & Phishing