Hi and welcome to Discussions,
here are Microsofts hints for update 'trouble':
Make sure that the Microsoft Office 2008 folder is located in the Applications folder on the startup volume. If it is at another location on your computer, move it to the Applications folder on your startup volume, and then try installing the update again.
Restart your computer and try installing the update again.
Make sure that the user account you use to authenticate the update installer is an administrator account. For more information about administrator accounts, see Mac Help.
Make sure that you downloaded the correct language version of the update for your copy of Office.
If you rename or modify any of the files in the Microsoft Office 2008 folder, you cannot install product updates. To correct this issue, drag the Microsoft Office 2008 folder to the Trash, and then reinstall Office 2008 from your original installation disk.
Quit any applications that are running, including all Office applications, Microsoft Messenger, and Office Notifications, because they might interfere with installation.
Maybe it helps
Stefan