Found this on the web, not my words, but it should fix the error.
While running setup for Office 2000, you may see a message, "Please
insert the Windows XP Professional CD into your CD-ROM drive," followed
by "Windows file must be installed for your installation to proceed.
Insert your Windows XP CD now." There is not an option to browse for the
needed file(s).
Cause:
This occurs because the Windows XP registry is forcing the installation
program to search an incorrect path for the needed files. It may affect
other Microsoft software other than Office 2000.
Resolution:
Edit the registry in the following manner.
1. Click on Start > Run > regedit. This will launch the registry editor.
2. Navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Curr
entVersion
3. Under CurrentVersion, click on the Setup folder. On the right side of
the screen, click on the Source Path key. Edit the value data to C:\.
4. Next, navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT
5. Under Windows NT, click on the CurrentVersion folder. Again, click on
the Source path key and edit the value data to C:\.
6. Close the registry editor and restart the computer. After doing so,
start the installation again.
Before he does this, I highly recommend he backs up the registry first.
I also found this Microsoft Knowledge Base
article
good luck