If you used the wrong product key to active a Windows XP computer, you can activate your computer with the correct product key using either of the following:

How do I perform an in-place upgrade (re-installation) of Windows XP?

Deploying Windows XP Using Windows Product Activation.

A simpler method, if only a few computers are involved, is:

01. Open a CMD.EXE window.

02. Using CrtRP.bat from tip 8975, type:

CrtRP "Prior to changing Product key" OK

03. Type If "%OK%" EQU "N" @echo Restore Point creation failed..

04. Type REG ADD "HKLM\Software\Microsoft\Windows NT\CurrentVersion\WPAEvents" /V OOBETimer /T REG_BINARY /F /D FF00FF00FF00FF00FF00FF00 and press Enter.

05. Type %systemroot%\system32\oobe\msoobe.exe /a and press Enter.

06. Select Yes, I want to telephone a customer service representative to activate Windows and press Next.

07. Press Change Product key.

08. Type the new product key and press Update. If you are returned to the previous window, press Remind me later and restart your computer.

09. To verify that you are activated, type %systemroot%\system32\oobe\msoobe.exe /a and press Enter. You should receive:

Windows is already activated. Click OK to exit.

10. Press OK.