The Klez virus is one that randomizes the return addresses. You
can't directly figure out who sent it. All you know is:
1. It's not the "apparent" sender, and
2. Both you and the "apparent" sender are in the infected person's
This strain is particularly bad, as some of its messages claim that
the attachment itself will clean your computer of the virus.
Zoli and Erika are correct--time to be extra vigilant and make sure
your anti-virus program is current.