On Tuesday, Microsoft released Office 2004 11.5.7 update for the Mac. This update applies to Office 2004 Standard Edition, Office 2004 Student and Teacher Edition, Office 2004 Professional Edition, Word 2004, Excel 2004, PowerPoint 2004, Entourage 2004. If you're still using this aging version of Office,* you'll want to grab this update.
According to Microsoft, the update fixes a vulnerability that could allow someone to overwrite the contents of a computer's memory with their own code. For complete information on the vulnerability and the fix in this update, look here and here.
The first issue, described in Microsoft Security Bulletin MS10-003, notes that an Office file containing code that exploits the vulnerability could gain full user rights to a machine when opened by a user, while the issue described in Microsoft Security Bulletin MS10-004 is specific to PowerPoint.
Note that this update requires Mac OS X 10.2.8 (Jaguar) or a later and the above Office products.
*I'm still using Office X, which was released way back in 2001, so don't feel badly.