Microsoft has just released updates to the 2004 and 2008 versions of Microsoft Office. According to the company, the updates, which weigh in at 9.7 MB and 221.5 MB respectively, provide "fixes for vulnerabilities that an attacker can use to overwrite the contents of your computer's memory with malicious code" as well as improvements to stability and performance.
As with most Mac OS X-related updates, whether from a third party app or a system update, you should consider backing up your data before proceeding.
Keep in mind, as you install this update, that right around the corner is Microsoft Office for Mac 2011. Notable updates for the next version of the suite for the Mac include the replacement of Entourage with Outlook, the return of VBA and, gasp (or hurrah!), a more ribbon-oriented user interface, à la the Windows version of Office.