Office 97 Serial Number Won’t Change…

I have some PC’s that once had Office 97 installed on them. Later, we un-installed Office from them but now I need Office 97 re-installed on them. When I do, however, the serial number from the prior installation of Office 97 still shows on the splash screens when any Office Program (Word, Excel, etc.) is opened. I have searched the Registry and found the old number referenced once or twice to which I changed but hasn’t made any difference (even after a restart). Any ideas on what I can do to get this fixed?