Stu6723,
You are permitted to reinstall and activate XP as many times as you want to, as long as your use of XP maintains compliance with the End User Licensing Agreement (EULA).
After a certain number of activations within a certain time period, automatic online activation will be suspended, which will require that you do a Telephone Activation. To do so, rerun the Activate Windows Wizard and select "Activate my product using the telephone". Follow the phone prompts.
If the automated telephone activation is not accepted, you will be offered the option to speak with a live activation representative. Choose that option. The rep will ask a few questions for the purpose of confirming that you are still using your license for XP in accordance with the EULA. If so, the rep will issue an activation code for you to enter into the Activation Wizard.