I think I found the answer:
Run Setup for Office XP.
Type the Product Key in the User Information dialog box, and then click OK .
In the Product Compliance Check dialog box, click the Drive list, and then select your CD-ROM drive. You may need to wait if your CD-ROM spins up.
Eject the Office XP CD, and then insert the Office 97 Small Business Edition CD. You may need to wait for the operating system to read the CD. To do this, wait for the CD-ROM to finish spinning up.
Click OK in the Product Compliance Check dialog box.
When the Office XP End-User License Agreement appears, eject the Office 97 Small Business Edition CD, and then insert your Office XP Upgrade CD.
NOTE : If you do not eject your Office 97 Small Business Edition CD at this point, you receive an error message after the Office XP installation continues.
For additional information, click the article number below to view the article in the Microsoft Knowledge Base:
Q300852 OFFXP: Error 25003 During Upgrade Installation
Accept the End-User License Agreement, and then click Next .
Complete the installation for Office XP.