You will be given the option of registering/activating online or by phone. If you have a modem installed you can try it online. Depending upon what you have installed or changed, that is the fastest method and doesn't require typing in any numbers. However, if you have made major changes in the system then you will most likely be "rejected" if you try to do this by modem; in which case you will be given a 24 hour phone number and a set of numbers to "punch in" over the phone or read off to a MS person who will in turn give you another looooooong set of numbers to punch in to activate your copy of XP.