The CD does not contain a product key.  Your CDs (if they came with the PCs) are known as OEM CDs.  The CD will generally work ONLY with PCs by that manufacturer, i.e. Dell.  Basically speaking, you can use any one of the XP CDs with either of the PCs from Dell that have a XP PID label on the PC.  Use the number on the label when installing XP. Same goes for Windows 2000.