Windows Product Activation after System Repair
After running a repair, do I have to re-activate Windows XP?
Yes. The subject behavior is by design.
The repair process replaces the system license file. To workaround this behavior, you can save the Wpa.dbl file after activation and replace the Wpa.dbl file with your saved copy after a repair.
This file is located in the %systemroot%\system32 folder.