VegasRob,
Most people in your situation spend less than 3-5 minutes reactivating XP by phone, assuming that you have been installing/using XP in accordance with the End User Licensing Agreement.
Windows Product Activation checks hardware periodically to see if it is still installed on the same collection of hardware as it was during its last activation. It can tolerate a few changes but if there are too many, you are required to reactivate.