When you boot Windows XP, you must activate it. When you do this, it makes a hardware ID for your computers hardware configuration, and takes your CD key, and sends all this to Microsoft.If the key has never been used before, or it's been used on this same hardware, it becomes activated.
When you enter the product key do you get an error message? - the product key entered is not valid.If that is the case and you have a legit copy of Windows Xp., it is likely that you mistyped the product key. The product key is printed in a machine-readable font in which "B"s and "8"s often look similar.
The software you are trying to install might have exceeded the allowed number of activations, if that's the case you will have to contact Microsoft, to have the activation count reset.
If the product is already registered on a different machine, you may be trying to install a second copy of the product when it is not allowed. When you first activate a Microsoft product, the activation software creates a "hash"1 for your computer. If you attempt to install the software on a computer that generates a different hash, either because you have materially changed the hardware in your computer or because it is a different computer altogether, the activation attempt will fail.
If you have upgraded your hardware, or are permanently moving the software from one computer to another, you must call Microsoft in order to reset your activation information.
Good luck, I hope you get XP, registered.
Winnipeg