The CD key would not be on the disc if it was legitimate OEM / Retail media.
Actually there is a CD key located in the "unattend.txt" file on every Windows XP CD but it's not going to activate with Microsoft, it's used for deployment of Windows installations. OEM's typically image a system and then copy the images to multiple machines with identical hardware, they don't sit through and install it on every machine. The Windows setup requires a key to install, even in unattended mode so a 'Generic Key' is used to get through setup which is what is located on the disc. When you fist boot into windows you are asked for the key on the COA which is what is used to activate with Microsoft.