Backup your Registry/System State

Backup your system state by clicking Start > Run > and typing ntbackup > Click the Advanced Mode button in the Backup Utility Wizard. >Click the Backup tab, then in Click to select the check box for any drive, folder, or file that you want to back up, select the System State.
As an alternative, you can backup just the Registry by clicking Start > Run > and type in Regedit From within the Regedit screen, right click My Computer, choose Export, name the file whatever you choose, and click Save
To change the product ID

Log in as the local Administrator
Click Start > Run > and type in Regedit
Browse to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\wpaevents


In the right pane, right-click OOBETimer, and then click Modify
Change at least one digit of this value to deactivate Windows
Click OK and close regedit
Click Start > Run and type in: "%systemroot%\system32\oobe\msoobe.exe /a"
Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next
Click Change Product Key (at the bottom)
Enter your valid Corporate Product Key
Press Update and close the window.
If you are returned to the previous window, click Remind me later
Restart your computer
Verify the change

After the workstation restarts, click Start > Run
Type in: "%systemroot%\system32\oobe\msoobe.exe /a" without the quotes.
Make sure the dialog box says 'your copy of windows is already activated'
If you performed the above steps incorrectly, or used an invalid key, your system may not be able to boot. Use the F8 key to boot to the last known good configuration and retry with a valid key.

Troubleshooting
If you tried the above steps and nothing happens:

Make sure you are logged in with the local administrator account, not just an account with Administrator privileges.
Try replacing the %systemroot% variable with the actual drive letter that your actual directory path, especially when dual booting, or if the system path is on a drive other than C:\
On a normal Windows XP installation, your systemroot should be C:\windows\ so the command should be C:\Windows\system32\oobe\msoobe.exe /a
On systems upgraded from Windows NT/2000, the systemroot directory may be C:\Winnt\ so the command should be C:\winnt\system32\oobe\msoobe.exe /a
Make sure you include a space between .exe and /a in the command:
Correct - C:\winnt\system32\oobe\msoobe.exe /a
Incorrect - C:\winnt\system32\oobe\msoobe.exe/a
Product keys for XP Home Edition and retail versions of XP Professional will not activate a corporate or OEM version of XP Professional. The algorithms are different.
If you do not have a valid installation key for Windows XP, DO NOT e-mail us asking for a key, keygen or crack.

http://www.labmice.net/WindowsXP/articles/changeID.htm

To view your current XP Key, use this program.

In this program, you can go to Options and use it to change your XP product key.

To find the most recent version of this software, visit: http://www.magicaljellybean.com/keyfinder.shtml