Hello,
One of my friends has just had exactly the same problem. He had hundreds of bounced e-mails in his inbox.
There are a couple of viruses out which do this, recently one has become "popular"!
This one does it, for example,
http://www.f-prot.com/news/vir_alert/sobig_e.htmlThere are a couple of others.
Unfortunately, you can't stop people sending mail which appears to be from you.
Just install a virus checker, get everyone you know to also install a virus checker. Apart from that, there's nothing you can do.