Sometimes you have the need to change your windows XP Serial number, wether because you had an illegal copy and bought a license, or you are ghost-installing a bounch of similar computers and you want to install each's individual key.

There are two methods I know of for doing this:

Method 1: Microsoft's Official Tool

Microsoft's Official Windows Product Key Update Tool (for changing keys only):
http://go.microsoft.com/fwlink/?LinkId=50346&clcid=0x409

Method 2: The manual way

 1. Click "Start" and click "Run".  Then type "%systemroot%\system32\oobe\msoobe.exe /a"  (Without the quotes).

2. Select  "Yes, I want to telephone a customer service representative to activate Windows" then click "Next.

3. Click "Change Product Key."  This will allow you to enter a new valid windows XP Serial.

4. Click "Update" and close the window.

5. Restart your computer.   Once rebooted, your XP Should be installed with the new serial number.