Microsoft Excel XP and 2003 for Windows and Excel for Mac contain a flaw that could be used by an attacker to take control of your computer. This can be accomplished by convincing the computer's user to simply open a specially crafted Excel document, including documents hosted on web sites or attached to email messages. Check with the sender if you are concerned about the validity of a message as the e-mail from: field can be forged so that it appears that an e-mail is coming from someone you know or even from yourself. In order to protect yourself, you should be extremely cautious about opening any attachments or links.

Further information is available at:
http://www.us-cert.gov/cas/techalerts/TA06-167A.html
http://www.kb.cert.org/vuls/id/802324