When installing XP on a Dell system with a Dell XP Reinstalllation CD and the setup is run via the BIOS no product key is required. The installation will use a Dell System Locked Preinstallation (Dell SLP key) and automatically activate. See my wiki A Clean Install of Windows XP for some more details on installing Windows XP.
The serial number and Microsoft XP COA have rubbed off the labels on the bottom of my netbook. I have the service tag and all the numbers printed on the outside of the product box (PPID and various order numbers). Is there anyway to re-discover the COA as the hard disc has failed and I want to replace it and  re-install XP. Thanks KiraTW
Recovering COA & Serial Number