From: geonque on
On or after April 14 the office compatibility pack function ceased to work,
attempts to open or save *.docx files were greeted with an error: “unable to
open file”.

Attempted fix by:
uninstalling and re-installing compatibility pack
detect and repair on Office 2000

current versions reading:
Microsoft Word 2000 (9.0.8968 SP-3)
Windows 2000 5.00.2195 Service Pack 4

Relevant update history:
Windows 2000 Security Update for Windows Media Player 9 for Windows 2000
(KB979402) Wednesday, April 14, 2010
Office 2007 Update for the 2007 Microsoft Office System (KB981715)
Wednesday, April 14,2010
Windows 2000 Windows Malicious Software Removal Tool - April
2010(KB890830) Wednesday, April 14, 2010
Windows 2000 Security Update for Windows 2000 (KB981350) Wednesday, April
14, 2010
Windows 2000 Security Update for Windows 2000 (KB977816) Wednesday, April
14,2010
Windows 2000 Security Update for Windows 2000 (KB979683) Wednesday, April
14, 2010
Windows 2000 Security Update for Windows 2000 (KB978601) Wednesday, April
14, 2010
Windows 2000 Security Update for Windows 2000 (KB979309) Wednesday, April
14, 2010
Windows 2000 Security Update for Windows 2000 (KB980232) Wednesday, April
14, 2010
Windows 2000 Cumulative Security Update for Internet Explorer 6 Service
Pack 1 (KB980182) Monday, April 05, 2010

It seems likely the problem is with the Update for the 2007 Microsoft Office
System (KB981715).

Can anyone confirm a similar problem and/or suggest a fix?
From: abidman on
I can absolutely confirm what you are seeing.
It is update 981715 on the following config:
Windows 2000 SP4
Word Viewer 2003
Office 2007 Compat pack.

Prior to this update, I can open .docx files w/ no problem.
After the update, I get the error that you mentioned.

The update replaces 1 file - mso.dll (http://support.microsoft.com/kb/981715)

If I put an old version of that file back after installing the update, I can
then open the .docx files again.

I'm still looking for the best way to remedy the machines on which this
update got installed and would be glad to hear what others have done.