If you installed a second copy of Windows XP from your 1st CD-ROM, you have to change the the Product ID to activate the second copy, instead of reinstalling with the new media.
To change the Product Identification number:
1. Start / All Programs / Accessories / System Tools /Activate Windows.
2. In the Windows Product Activation Wizard, select Activate Windows by phone.
3. Press Change the product key and type the new Product ID.
4. Press Activate over the Internet.
NOTE: See Q328874 for the SP1 changes.