Sroop16,
Are you seeing a message along the lines of "activated too many times?"
Product Activation is set to temporarily suspend your Product Key if it is used more frequently or more times than "normal." However, as long as you are in compliance with the EULA, you are authorized to activate XP an unlimited number of times.
Select the option to activate by telephone. The phone reps will see the too many times flag, verify that you are using XP in accordance with the EULA, then release the suspension or issue a telephone activation code. Most people say this takes 3-5 minutes on the phone