Dell XP Recovery CD if you boot from it wont ask you for a product key. The COA on the machine being XP wont need to be used but is required as proof of licence.  
Good Afternoon!  Last week I purchased a Dell OptiPlex GX280 from a Computer Recycling organization here in Louisiana. I recently recieved the recovery disks from Dell that contains Windows XP Professional SP3. There is a Product License Sticker on the top of my computer with a license code on it. When prompted during setup, do I use the code on the machine? 
Reuse Product Code From Computer