Update: Microsoft has released another update for Office 2004 (version 11.6.2) which corrects this problem.
On Tuesday, Microsoft released updates for both Microsoft Office 2004 and 2008 to correct some security vulnerabilities in the software, but a number of people are having difficulties opening some Excel spreadsheets in Office 2004 after the update.
As mentioned by CNET member and MacFixIt reader Kurt in the comments of our article announcing the update:
"After applying the update yesterday, some Excel documents refuse to open. For a fraction of a second the open progress bar shows up, then nothing happens."
The current recommendation if you are experiencing this problem is to downgrade to the previous version of Office so the documents can still be opened.
You can downgrade Office in two ways. The first is to use a Time Machine backup of the whole system if you created one immediately before installing Office (we always recommend doing this). This method is preferred because it will set up your system exactly how it was before the problematic software was installed.
The second option is to fully uninstall Office and then reinstall it but only updating to the previous version of the program (version 11.6.0). An uninstaller utility should be located in the /Applications/Office/Additional Tools/Remove Office/ folder, which you can run to remove the software from your system. After this is done, reinstall from the installation DVDs and download and install the Office 11.6.0 update from Microsoft's Support page.
Update: Microsoft has released another update for Office 2004 (version 11.6.2) which corrects this problem. The latest version can be downloaded via Microsoft's Auto-Update utility or from the Microsoft Mac Support site.