Then, when you install WinXP you'll be prompted to activate it. Agree to the activation, and when it fails to happen via the internet (because most people haven't got around to setting up their connection so soon after an install) you'll be asked if you want to activate by phone. Say yes, and you'll be presented with the phone numbers to use - try the 'ordinary' one first, rather than the 0800 number. You'll be talked through the activation prosedure from then on.
Why on earth did you email Bill Gates about this - you didn't seriously expect a response did you?