For XP Installation Key problems:
Go to HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
In the right pane, right-click OOBETimer, and then click Modify.
Change at least one digit of first line value to deactivate Windows.
Run %systemroot%\system32\oobe\msoobe.exe /a
Preinstalled xp system use the key on the back of the machine.
All others use the same key, that you used to install windows xp
Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.
Click Change Product key.
Type the product key in the New key boxes, and then click Update.
If another Screen pops up-Click Remind me latter.
Reboot.
Then activate.
Reboot.
Install xp sp2 if you want too.