Microsoft has issued an update for Office 2004 for the Mac. This critical patch fixes a bug involving a malformed string that could allow an attacker to overwrite the contents of your machine's memory with malicious code.The 11.3.8 update also appears to address some stability issues in the Office suite. This update applies to Office 2004 Standard Edition, Office 2004 Student and Teacher Edition, Office 2004 Professional Edition, and Word 2004.
You can read more on this particular vulnerability from this Microsoft Security Bulletin.
You can download the update directly from Microsoft's download site here.