Changing Windows XP Product Key Windows XP computers must all have valid product keys and be registered with Microsoft through the Windows Product Activation process. Illegal or invalid product keys will prevent the application of service packs and hotfixes. Without delving into the reason why you might have a computer with an invalid product key, we will assume for the sake of this tip that you now own a valid and legal copy of Windows XP, and would like to properly register it without having to completely reinstall the operating system. Fortunately, there is not too difficult if you're not afraid to delve into the Registry. Start by changing one of the values of the Registry Key specified below to any number 0-9 or character A-F. Changing any character will do - it doesn't matter which one you change or what you change it to.

Hive: HKEY_LOCAL_MACHINE
Key: Software\Microsoft\WindowsNT\CurrentVersion\WPAEvents
Name: ODBETimer
Data Type: REG_BINARY
Value: just change one of the entries as mentioned above What this will do is deactivate the copy of Windows.
Now click Start, Run, and copy the following into the Run box: %systemroot%\system32\oobe\msoobe /a

This launches the Product Activation Wizard. From there you can choose an option such as calling a Microsoft representative, and select the Change Product Key option. Enter your new product key, Update and close the Wizard and reboot (you may have to select Remind Me Later if the Wizard pops up again). When you reboot, follow the normal activation process which should work if you've entered a valid product key.