There have been a few threads on this already and there are bound to be more to follow.
If you are receiving (as i am) loads of returned undeliverable e-mails from people you don't know DELETE THEM.
They are fall out from the recent release of the Gibe-F (or Swen) Worm click here
The worm will pick your e-mail address up from someone else's PC who has the worm and will send out copies of itself using your e-mail address as the return address. Hence when the message is rejected because its infected YOU get the Undeliverable Message,
It does not mean you are infected.
Make sure your AV is up to date or get free protection from click here AVG 6 Free Edition.