Escamillo suggests:

Add a new e-mail address to your list, named !!!000. This is an invalid address but Outlook Express will allow you to enter it. It is automatically placed first in the list. As soon as a virus opens the list and tries to start sending mails, it produces the error "invalid address" and is stopped.

Not true. It might work on some viruses but it doesn't really protect you much: lots of viruses bypass that - for instance some have their own SMTP client code so MS outlook plays not part in the sending of copies.

Anahata