Yikes. This one certainly sounds scary. It apparently doesn't affect Excel 2007, so it pays to be an early adopter in this case. Just rely on the old adage: never open attachments from email addresses you don't know.

"If a rigged .xls document is launched, the exploit happens silently in the background, infecting the machine with a Trojan downloader that opens a backdoor and waits for instructions from a server controlled by the attacker."