How to activate Windows XP professional?

I have a genuine copy of windows XP professional and a key that has NEVER been linked onto any other pc. I recently installed that copy onto a laptop, and there were no issues during the installation; after the installation was complete and the laptop was running, it asked me to activate windows (which i have no problem doing because its a legit and genuine copy)... every time I input the license key and click retry, the windows just keeps popping back up and never activates. Any Ideas besides activating it over the phone?