When you reinstall it you will have to re-activate it
If you reinstall the OS on the same computer, you'll need to reactivate. Since the hardware hasn't changed, you can reactivate through the Internet.
Windows XP, WPA creates a nonunique value based on up to ten pieces of information from your video card, network card, SCSI controller, hard drive, CPU, and memory configuration. The tool then uses a combination of the 25-digit product ID code and the nonunique value to create a number, which Microsoft exchanges for a code that activates your copy of the OS.