How to change the serial number used in Windows XP

Step 1

Open Start/Run... and type the command:
regedit and click "OK" (or press ENTER).
Go to HKey_Local_Machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents,
on the right double click on "oobetimer" and change at least one digit of this value to deactivate windows.
Click "OK" and close the Registry Editor.

Step 2

Open Start/Run... and type the command:
%systemroot%\system32\oobe\msoobe.exe /a and click "OK" (or press ENTER).
This will bring up the "Activate Windows" window.
Check the option for "Yes, I want to telephone a customer service representative to activate Windows" and click "Next"

Step 3

Then click "Change Product Key" (don't enter any information on that screen)

Step 4

Type in the new key and click "Update"
The activate Windows by phone window will reappear at this point, just close it by clicking the X in the upper right hand corner

Step 5

Reboot your system and Open Start/Run... and type the command:
%systemroot%\system32\oobe\msoobe.exe /a and click "OK" (or press ENTER).
If you see "Windows is already activated" then everything is OK.