The screen you should see now is the Edit Binary Value window with OOBETimer in the "Value name:" field.
As part of the process to change your Windows XP product key, you'll need to deactivate Windows XP. Deactivating Windows XP is accomplished by changing the value of OOBETimer, something you're about to do.
Select any part of the OOBETimer value by double-clicking on it.
Note: I've distorted much of the hexadecimal series for OOBETimer in this and other screenshots but you'll see several letters and numbers on your computer.