The quickest way to start this is to press ⊞ Win+R and type oobe/msoobe /a. If you haven't activated yet, you'll likely be prompted to enter your 25-character product key. You can find this key with your Windows XP disc or on a sticker affixed to your computer's case. If you have an active internet connection, activating online is the quickest way. You can also activate using a dial-up modem. If you've used this XP key on a different computer in the past, you may have to contact Microsoft by phone to activate Windows XP. If you don't have an internet connection, you can activate Windows XP by calling the Microsoft activation center. Give the support rep the installation ID that is displayed in the activation wizard, and then enter in the code that the support rep gives to you. After entering the code, Windows XP should activate.

Summary:
Launch the Activate Windows wizard. Enter your product key. Activate over the internet. Activate over the phone if you don't have internet.