Ensure that BEFORE you install Cognos for MS Office, that you run the MS KB Patch that came with it.
So your steps...
1. Uninstall CfMSO
2. Run the MS KB Patch.
3. Re-install CfMSO
I believe the MS KB Patch is in the "<Extracted_folder>/go_office folder. The KB Patch folder is KB908002. Run all three executables in this folder.
extensibilityMSM.msi
lockbackRegKey.msi
office2003-kb907417sfxcab-ENU.exe