During the installation of Windows XP Professional, you are prompted to enter a 25 digit Windows XP Product Key, which Windows XP promptly converts it into the system's product ID. Because of security concerns about piracy, Microsoft does not provide a tool that allows you to view the Product Key (or CD Key) that was used to install the operating system. However, Windows XP Service Pack 1 ships with a list of the two product IDs that are created by the pirated product volume license product keys.
The Product ID can be found by right clicking My Computer and choosing Properties.

To determine eligibility for the update, Service Pack 1 compares the Windows XP product ID on the system to this list.
The comparison and the list reside locally on the users PC and no information is sent to Microsoft as part of this process. Service Pack 1 for Windows XP will fail to install on installations of Windows with one of the following product IDs: XXXXX-640-0000356-23XXX and XXXXX-640-2001765-23XXX
The following message will be displayed if installation fails for this reason:

Service Pack 1 Setup Error: The product key used to install Windows is invalid. Please contact your system administrator or retailer immediately to obtain a valid product key. You may also contact Microsoft Corporation's Anti-Piracy Team by emailing piracy@microsoft.com if you think you have purchased pirated Microsoft software. Please be assured that any personal information you send to the Microsoft Anti-Piracy team will be kept in strict confidence.

If the product ID matches the invalid keys above, you may need to change the key (re-enter a valid key) in order to install Windows XP service pack 1, and to make sure your environment is legal. You could completely re-install Windows XP Professional or you can try the method below. Backup your system before attempting this.

This workaround is only for the corporate editions of Windows XP Professional using a compromised or illegitimate key. Windows XP Home Edition and retail versions of XP Professional are not affected by Service Pack 1.
Although this procedure may work with other versions of XP, it has only been tested on the corporate edition (volume license version) of Windows XP Professional.

Note: Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs.

Backup your system state by

1. Click Star, Run and type ntbackup
2. Click the Advanced Mode button in the Backup Utility Wizard.
3. 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.

1. Log in as the local Administrator
2. Click Start , Run and type in Regedit
3. Browse to:

   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\wpaevents

4. In the right pane, right-click OOBETimer, and then click Modify
   
5. Change at least one digit of this value to deactivate Windows
6. Click OK and close regedit
7. Click Start , Run and type in:

   %systemroot%\system32\oobe\msoobe.exe /a

8. Click "Yes, I want to telephone a customer service representative to activate Windows, and then click Next
9. Click Change Product Key (at the bottom)
10. Enter your valid Corporate Product Key
11. Press Update and close the window.
12. If you are returned to the previous window, click Remind me later
13. Restart your computer

After the workstation restarts,

1. Click Start, Run and Type in:

   %systemroot%\system32\oobe\msoobe.exe /a

2. Make sure the dialog box says 'your copy of windows is already activated'

• 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: