NB,
To (re)enter a M$ Word serial number, it is not necessary to reinstall Word.
This most always is the solution:
1. Quit all M$ applications.
2. Remove the following:
A. ~/Library/Preferences/Microsoft/Microsoft Office Settings (11).
B. /Applications/Microsoft Office 2004/Office/OfficePID. NOTE: This is normally an invisible file, so you may need a utility like FileBuddy to make the file visible so that it can then be deleted.
3. Relaunch Word and reregister.
P.S.
FWIW: A bit of background-
With people getting new machines, creating home networks, and using Migration Assistant to update their new machine from an older one, the M$ serial number issue is really quite common.
When Office is run, M$ (like other developers) checks for duplicate serial numbers on the network. Even though the user has a (new) valid serial number to enter, he can’t register it because he copied (used) the old serial number when he used MA. Ergo, the “solution” above.