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.

Links

Frequently Asked Questions about Microsoft Product Activation – 302878
Windows XP frequently asked questions about Windows Product Activation (WPA)
Microsoft Product Activation Frequently Asked Questions General Questions
Microsoft Product Activation: Frequently Asked Questions
Piracy Basics: Microsoft Product Activation
Inside Windows Product Activation