Just had the same problem after using monolingual to strip out unwanted
language files and code. However, after spending a few minutes looking at the
update package I have been able to find an effective solution that does not
require reinstallation. Here goes:
1) Download the 12.1.1 update directly from Microsoft's Mactopia website.
2) Mount the downloaded DMG file and copy the installer to your desktop.
3) Right-click the installer and select "show package contents"
4) Navigate to Office 2008 12.1.1
Update/Contents/Resources/package_updatable and open the file in TextEdit
5) Scroll to the bottom and find the line "found_valid_version=False"
6) Change False to True
7) Then delete all the text below until the start of "if not
found_valid_version:" Making sure that line is not deleted!
8) Close the window, it will ask if you want to overwrite the file. Say yes.
9) Close the finder window and then run the installer like normal.
Hopefully it should then work. Seems the problem was with the checksum - by
deleting that the installer goes without a problem.
Hope you find that useful!