I've found that the instructions on Microsoft's site to "... install all High-Priority updates from Microsoft Update before downloading the Compatibility Pack." are very important => if you install the Compatibility Pack before doing so, it will not work correctly ... and you can't then simply apply the updates to resolve it.
If that was the case here, I'd completely uninstall Office; then reinstall it; do all updates; and THEN install the compatibility pack. If done in the correct order, I've seen no issues with .DOCX files under Office 2003.
Note also that there have been several updates that apply to the compatibility pack => be sure you run Microsoft Update (or Office Update if you're still using Windows Update) AFTER installing the compatibility pack to ensure those are all installed as well.
