This Klez virus is far and away the most insidious virus I've seen. It masquerades itself in dozens of ways. If you get it in your system, you'll probably have to reinstall your operating system from scratch. I know; I stupidly clicked on an attachment and have spent days reinstalling.
Look at this site http://www.viruslist.com/eng/index.html for details on the Klez virus.
Do not open email attachments without being absolutely sure that it's not infected!
AlanF